Add datetime support - #36

`MS_CONSTR_STR_REGEX` slot clear was missing. It was not properly
traversed and freed. Now it is.

It used to fail with memory sanitized with:

```
 ==2637==ERROR: AddressSanitizer: attempting free on address which was not malloc()-ed: 0x50e000e8e350 in thread T0
    #0 0x7f07a10fc4d8 in free ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:52
    msgspec#1 0x7f079c42ddf4 in TypeNode_Free src/msgspec/_core.c:3474
    msgspec#2 0x7f079c42dbdf in TypeNode_Free src/msgspec/_core.c:3468
    msgspec#3 0x7f079c55e2eb in JSONDecoder_dealloc src/msgspec/_core.c:17235
    msgspec#4 0x7f07a0999132 in _PyEval_EvalFrameDefault Python/bytecodes.c:1398
    msgspec#5 0x7f07a0a15f73 in _PyObject_VectorcallTstate Include/internal/pycore_call.h:92
    msgspec#6 0x7f07a0a15f73 in method_vectorcall Objects/classobject.c:69
    msgspec#7 0x7f07a098d82f in _PyEval_EvalFrameDefault Python/bytecodes.c:3263
    msgspec#8 0x7f07a0a14e6c in _PyFunction_Vectorcall Objects/call.c:419
    msgspec#9 0x7f07a0a14e6c in _PyObject_FastCallDictTstate Objects/call.c:144
    msgspec#10 0x7f07a0a14e6c in _PyObject_Call_Prepend Objects/call.c:508
    msgspec#11 0x7f07a0ac3381 in slot_tp_call Objects/typeobject.c:8791
    msgspec#12 0x7f07a0a14206 in _PyObject_MakeTpCall Objects/call.c:240
    msgspec#13 0x7f07a098d20a in _PyEval_EvalFrameDefault Python/bytecodes.c:2715
    msgspec#14 0x7f07a0a14e6c in _PyFunction_Vectorcall Objects/call.c:419
    msgspec#15 0x7f07a0a14e6c in _PyObject_FastCallDictTstate Objects/call.c:144
    msgspec#16 0x7f07a0a14e6c in _PyObject_Call_Prepend Objects/call.c:508
    msgspec#17 0x7f07a0ac3381 in slot_tp_call Objects/typeobject.c:8791
    msgspec#18 0x7f07a0a1550e in PyObject_Call (/opt/hostedtoolcache/Python/3.12.15/x64/lib/libpython3.12.so.1.0+0x21550e) (BuildId: d5ce433e66c26342bfaf6b82ab1a95541a8ebade)
    msgspec#19 0x7f07a098d82f in _PyEval_EvalFrameDefault Python/bytecodes.c:3263
    msgspec#20 0x7f07a0a14e6c in _PyFunction_Vectorcall Objects/call.c:419
    msgspec#21 0x7f07a0a14e6c in _PyObject_FastCallDictTstate Objects/call.c:144
    msgspec#22 0x7f07a0a14e6c in _PyObject_Call_Prepend Objects/call.c:508
    msgspec#23 0x7f07a0ac3381 in slot_tp_call Objects/typeobject.c:8791
    msgspec#24 0x7f07a0a14206 in _PyObject_MakeTpCall Objects/call.c:240
    msgspec#25 0x7f07a098d20a in _PyEval_EvalFrameDefault Python/bytecodes.c:2715
    msgspec#26 0x7f07a0a14e6c in _PyFunction_Vectorcall Objects/call.c:419
    msgspec#27 0x7f07a0a14e6c in _PyObject_FastCallDictTstate Objects/call.c:144
    msgspec#28 0x7f07a0a14e6c in _PyObject_Call_Prepend Objects/call.c:508
    msgspec#29 0x7f07a0ac3381 in slot_tp_call Objects/typeobject.c:8791
    msgspec#30 0x7f07a0a14206 in _PyObject_MakeTpCall Objects/call.c:240
    msgspec#31 0x7f07a098d20a in _PyEval_EvalFrameDefault Python/bytecodes.c:2715
    msgspec#32 0x7f07a0a14e6c in _PyFunction_Vectorcall Objects/call.c:419
    msgspec#33 0x7f07a0a14e6c in _PyObject_FastCallDictTstate Objects/call.c:144
    msgspec#34 0x7f07a0a14e6c in _PyObject_Call_Prepend Objects/call.c:508
    msgspec#35 0x7f07a0ac3381 in slot_tp_call Objects/typeobject.c:8791
    msgspec#36 0x7f07a0a14206 in _PyObject_MakeTpCall Objects/call.c:240
    msgspec#37 0x7f07a098d20a in _PyEval_EvalFrameDefault Python/bytecodes.c:2715
    msgspec#38 0x7f07a0ae8f35 in PyEval_EvalCode Python/ceval.c:580
    msgspec#39 0x7f07a0ae7a3c in builtin_exec_impl Python/bltinmodule.c:1093
    msgspec#40 0x7f07a0ae7a3c in builtin_exec Python/clinic/bltinmodule.c.h:543
    msgspec#41 0x7f07a0a345a2 in cfunction_vectorcall_FASTCALL_KEYWORDS Objects/methodobject.c:438
    msgspec#42 0x7f07a0a14532 in _PyObject_VectorcallTstate Include/internal/pycore_call.h:92
    msgspec#43 0x7f07a0a14532 in PyObject_Vectorcall Objects/call.c:325
    msgspec#44 0x7f07a098d20a in _PyEval_EvalFrameDefault Python/bytecodes.c:2715
    msgspec#45 0x7f07a0ae8f35 in PyEval_EvalCode Python/ceval.c:580
    msgspec#46 0x7f07a0b09f76 in run_eval_code_obj Python/pythonrun.c:1757
    msgspec#47 0x7f07a0b09aaa in run_mod Python/pythonrun.c:1778
    msgspec#48 0x7f07a0b099f1 in pyrun_file Python/pythonrun.c:1674
    msgspec#49 0x7f07a0b09852 in _PyRun_SimpleFileObject Python/pythonrun.c:459
    msgspec#50 0x7f07a0b09667 in _PyRun_AnyFileObject Python/pythonrun.c:78
    msgspec#51 0x7f07a0b12943 in pymain_run_file_obj Modules/main.c:361
    msgspec#52 0x7f07a0b12943 in pymain_run_file Modules/main.c:380
    msgspec#53 0x7f07a0b12943 in pymain_run_python Modules/main.c:634
    msgspec#54 0x7f07a0b12943 in Py_RunMain Modules/main.c:714
    msgspec#55 0x7f07a0b1243a in Py_BytesMain Modules/main.c:768
    msgspec#56 0x7f07a042a1c9  (/lib/x86_64-linux-gnu/libc.so.6+0x2a1c9) (BuildId: a4a7992a8e66555c8141ab2a08a8465ff6e0ea65)
    msgspec#57 0x7f07a042a28a in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28a) (BuildId: a4a7992a8e66555c8141ab2a08a8465ff6e0ea65)
    msgspec#58 0x558976af7094 in _start (/opt/hostedtoolcache/Python/3.12.15/x64/bin/python3.12+0x1094) (BuildId: d664fd041bd9df9902c326eb80717fa04325aaaa)

0x50e000e8e350 is located 16 bytes inside of 152-byte region [0x50e000e8e340,0x50e000e8e3d8)
allocated by thread T0 here:
    #0 0x7f07a10fd9c7 in malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:69
    msgspec#1 0x7f07a0a7c2fc in gc_alloc Modules/gcmodule.c:2330
    msgspec#2 0x7f07a0a7c2fc in _PyObject_GC_NewVar Modules/gcmodule.c:2364

SUMMARY: AddressSanitizer: bad-free ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:52 in free
==2637==ABORTING
```

See
https://github.com/msgspec/msgspec/actions/runs/38243375356/job/114791328508

Now it is properly cleaned.