A few cleaning up for the mmtk GC module

[wks]

This pull request includes several commits that clean up the code, including:

• Removed an unused constant HAS_MOVED_GFIELDSTBL. The mmtk/mmtk-ruby repo has stopped using that flag a while ago, so we can remove it here, too.
• Updated the mmtk Rust dependency to the current latest version.
• Fixed all Clippy warnings (cargo clippy) and formatting issues (cargo fmt --check).
  • I also added a GitHub CI test for checking code styles.
• Rewrote some of the environment variable parsing code to use more idiomatic Rust style.

[wks]

ruby/ruby@f483bef#diff-db708b4117de648f1041678860fd9c891e3e45773eb1bcde4f4bd83c9558f6ed made changes to the file /gc/mmtk/mmtk.c directly in the ruby/ruby repo. The same change is not done in this repo. Therefore the GC::INTERNAL_CONSTANTS[:RBASIC_SIZE] is missing, causing some tests to fail.

[wks]

ruby/gc/mmtk/mmtk.c includes this line:

#include "internal/object.h"

If I compile the mmtk GC module inside the ruby repo, it will work.

make modular-gc install-modular-gc MODULAR_GC=mmtk MMTK_BUILD=debug

But if I copy mmtk.c from the ruby repo to the mmtk repo, goto the mmtk repo and compile it there, it will complain that it cannot include "internal/object.h". See https://github.com/ruby/mmtk/actions/runs/15314883809/job/43086759608?pr=36

> ~/projects/mmtk-github/parallels/ruby-master/ruby/build-debug/install/bin/bundle exec ~/projects/mmtk-github/parallels/ruby-master/ruby/build-debug/install/bin/rake install:debug
cd tmp/x86_64-linux/debug/3.5.0
/usr/bin/make
compiling ../../../../gc/mmtk/mmtk.c
../../../../gc/mmtk/mmtk.c: In function ‘rb_gc_impl_shutdown_call_finalizer’:
../../../../gc/mmtk/mmtk.c:1023:13: error: implicit declaration of function ‘RBASIC_RESET_FLAGS’ [-Wimplicit-function-declaration]
 1023 |             RBASIC_RESET_FLAGS(obj);
      |             ^~~~~~~~~~~~~~~~~~
At top level:
cc1: note: unrecognized command-line option ‘-Wno-self-assign’ may have been intended to silence earlier diagnostics
cc1: note: unrecognized command-line option ‘-Wno-parentheses-equality’ may have been intended to silence earlier diagnostics
cc1: note: unrecognized command-line option ‘-Wno-constant-logical-operand’ may have been intended to silence earlier diagnostics
make: *** [Makefile:264: mmtk.o] Error 1
rake aborted!
Command failed with status (2): [/usr/bin/make]
/home/wks/projects/mmtk-github/parallels/ruby-master/ruby/build-debug/install/bin/bundle:25:in 'Kernel#load'
/home/wks/projects/mmtk-github/parallels/ruby-master/ruby/build-debug/install/bin/bundle:25:in '<main>'
Tasks: TOP => install:debug => compile:debug => compile:debug:x86_64-linux => copy:debug:x86_64-linux:3.5.0 => tmp/x86_64-linux/debug/3.5.0/librubygc.mmtk.so
(See full trace by running task with --trace)

[peterzhu2118]

I fixed this issue on the main branch.

[wks]

All tests passed except Prism::RactorTest#test_parse_file, which failed in a way similar to #23