Use malloc based allocator

[soutaro]

Extracted from #2652

The initialization of the mmap based allocator impacts parsing performance, if the parsing input is small. Switching to malloc based allocator improved the parsing performance.

Details

Baseline with lexer fix

➜  rbs git:(fix-lexer) bundle exec ruby bin/benchmark-parse.rb core/**/*.rbs ../../ruby/gem_rbs_collection/gems/activerecord/8.0/*.rbs sig/**/*.rbs
Benchmarking parsing 177 files...
✅ 16.667 i/s (±0.000%)

malloc based allocator

➜  rbs git:(allocator) ✗ bundle exec ruby bin/benchmark-parse.rb core/**/*.rbs ../../ruby/gem_rbs_collection/gems/activerecord/8.0/*.rbs sig/**/*.rbs
Benchmarking parsing 177 files...
✅ 19.726 i/s (±0.000%)

[amomchilov]

For the record, this is essentially a revert of #38, back to the rudimentary allocator initially introduced in #2398