More carefully control build flags for underlying library #6

Build flags that might be interesting:

1. MPACK_DEBUG (should be on by default in debug/"ReleaseSafe", off by default in "ReleaseFast")
2. MPACK_EXTENSIONS (should be on by default)
3. MPACK_READ_TRACKING/MPACK_WRITE_TRACKING (should have similar defaults to MPACK_DEBUG)
4. MPACK_OPTIMIZE_FOR_SIZE (default off, on in "ReleaseSmall")

Flags that should probably be off by default (because we use Zig equivalents):

1. MPACK_MALLOC (we use Zig std.mem.Allocator)
2. MPACK_STDIO (we use Zig stdio bindings)

Also it might be interesting to disable parts of the API that aren't needed. Corresponding feature flags include: MPACK_WRITER, MPACK_READER, MPACK_NODE, etc...