When a CC is invalidated only the klass field is set to 0. After it's
invalidated it isn't safe to access the CME, as it may have been freed.

I made a similar change in Ruby 4.0 in
640a2f1dc77c0ecf226dbd71cf7a1eb876a1f037, but assumed it was due to the
changes we'd made to callcaches making klass a weak-reference.

Co-authored-by: Christian Bruckmayer <christian.bruckmayer@shopify.com>

to prevent the following scenario:

1. `delete_unique_str()` can be called while GC (sweeping)
2. it calls `st_insert()` to decrement the counter
3. `st_insert()` can try to extend the table even if the key exists
4. `xmalloc` while GC and cause BUG

When reference updating ObjectSpace.trace_object_allocations, we need to
check whether the object is valid or not because it does not mark the
object so the object may be dead. This can cause a segmentation fault
if the object is on a free heap page.

For example, the following script crashes:

    require "objspace"

    objs = []
    ObjectSpace.trace_object_allocations do
      1_000_000.times do
        objs << Object.new
      end
    end

    objs = nil

    # Free pages that the objs were on
    GC.start

    # Run compaction and check that it doesn't crash
    GC.compact

We need to reinsert into the ST table when an object moves because it is
a numtable that hashes on the object address, so when an object moves we
need to reinsert it rather than just updating the key.

We should be checking for key for moved objects rather than the value
because the key is a Ruby object and the value is malloc'd memory.

We observed crashes from rb_io_bufwrite() thread switching (through
rb_thread_check_ints()) in the middle of rb_execution_context_mark(). By
the time rb_execution_context_mark() gets a timeslice again, it read
garbage from a frame that was already popped in another thread, crashing
the process in SEGV. Other mark functions probably have their own ways
of breaking, but clearly, the usual IO code do too much for this
perilous pseudo GC context.

Use `FILE*` like before 5001cc47169614ea07d87651c95c2ee185e374e0
("Optimize ObjectSpace.dump_all"). Also, add type checking for
the private _dump methods.

Co-authored-by: Peter Zhu <peter@peterzhu.ca>

[Bug #20892]

Until the introduction of that method, it was impossible for a
Module name not to be valid JSON, hence it wasn't going through
the slower escaping function.

This assumption no longer hold.

This commit splits gc.c into two files:

- gc.c now only contains code not specific to Ruby GC. This includes
  code to mark objects (which the GC implementation may choose not to
  use) and wrappers for internal APIs that the implementation may need
  to use (e.g. locking the VM).

- gc_impl.c now contains the implementation of Ruby's GC. This includes
  marking, sweeping, compaction, and statistics. Most importantly,
  gc_impl.c only uses public APIs in Ruby and a limited set of functions
  exposed in gc.c. This allows us to build gc_impl.c independently of
  Ruby and plug Ruby's GC into itself.