Implement new StableApiDefinition GC methods

[ianks]

In order to support Truffleruby on magnus, we need to support GC operations, which currently relies on cruby internals.

/// Informs that there are external memory usages, so the GC can run more
/// often than it otherwise would if it was unaware of such allocations.
fn gc_adjust_memory_usage(&self, incr: isize);

/// Informs the GC that `young` is a new reference to `old`, allowing the
/// the old object to participate in generational GC.
fn gc_writebarrier(&self, old: VALUE, young: VALUE);

/// Opts out of generational GC and write barrier protection.
fn gc_writebarrier_unprotect(&self, obj: VALUE);