Improve build performance - #108

In this PR we improve build performance by two cheap fixes:

1. Only write changed files. The majority of files in output are unchanged and in a general physical filesystem, reading is faster than writing.
2. Reduce the parallelism to 4. When there are more workers around, the kernel overhead increases.

Build time:

Without cache (output is removed):

node ./scripts/generate-data.mjs  11.56s user 21.89s system 278% cpu 11.997 total

With cache

node ./scripts/generate-data.mjs  9.09s user 8.33s system 203% cpu 8.554 total

Compared with the benchmark result in #105, the build process is now 15% faster.