Updated GC signatures

[sampersand]

This pr updates GC's signatures, unit tests, and adds in the Writer test type (which implements _Writer).

Two methods, GC.latest_gc_info and GC.stat technically can accept any hash, and just add their respective keys/values (e.g. [K,V] (Hash[K, V]) -> Hash[K | Symbol, V | Integer]). However, this is currently not testable, and the added complication of supporting it isn't worth the very niche—but technically correct—signature.

In addition to removing the occasional leading ::, this has the following changes:

• GC::compact_info type alias was added for the return value of GC.compact and other methods
• GC::{INTERNAL_CONSTANTS,OPTS}: Moved inside the class
• GC.start: Removed duplicate argument names.
• GC.stat: Gave actual names to arguments, Hash variant is now nilable.
• GC.stress: bool not TrueClass | FalseClass
• GC.stress=: Split Integer and bool cases apart to fix return value.
• GC.verify_compaction_references: Added optional toward, double_heap and expand_heap keyword arguments
• GC.latest_gc_info: nil is now accepted, removed K from the Hash[K | Symbol, unty]ed function.
• GC.garbage_collect: Removed duplicate argument names.
• GC::Profiler was moved inside the class, and auto_compact-related functions were added.

[sampersand]

Oh lovely, ruby segfault: https://github.com/ruby/rbs/actions/runs/6256735966/job/16988021332?pr=1530