Optimized the loading of descriptors - #328

name                                 old time/op  new time/op  delta
LoadDescriptor_Upb                    112µs ± 1%    69µs ± 5%  -38.51%  (p=0.000 n=11+12)

    FILE SIZE        VM SIZE    
 --------------  -------------- 
   +16%    +174  [ = ]       0    [Unmapped]
  +0.2%      +5  [ = ]       0    upb/upb.c
     +36%      +5  [ = ]       0    _start
  -0.1%     -16  -0.1%     -16    upb/def.c
    +3.6%     +32  +3.8%     +32    create_enumdef
    +6.9%     +16  +8.3%     +16    upb_symtab_new
    -2.2%     -64  -2.2%     -64    create_msgdef
  -0.3%     -32  -0.4%     -32    bazel-out/k8-opt/bin/external/com_google_protobuf/google/protobuf/wrappers.upb.c
    -0.4%     -32  -0.4%     -32    [section .rodata]
  -0.5%     -32  -0.5%     -32    upb/decode.c
    -9.8%     -32  -9.8%     -32    [section .rodata]
  -1.2%    -107  -1.2%     -96    upb/table.c
    [NEW]    +643  [NEW]    +608    table_hash
     +68%    +128   +89%    +128    upb_strtable_init2
    +8.3%     +51  +8.5%     +48    upb_strtable_resize
    +2.2%     +48  +2.2%     +48    upb_inttable_compact2
    +2.9%     +32  +3.0%     +32    upb_inttable_insert2
    +6.4%     +16  +7.7%     +16    upb_inttable_init2
   -10.4%     -64 -11.3%     -64    upb_strtable_insert3
    [DEL]    -961  [DEL]    -912    upb_murmur_hash2
  -0.0%      -8  -0.1%    -176    TOTAL