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>

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.

[Feature #20205]

As a path toward enabling frozen string literals by default in the future,
this commit introduce "chilled strings". From a user perspective chilled
strings pretend to be frozen, but on the first attempt to mutate them,
they lose their frozen status and emit a warning rather than to raise a
`FrozenError`.

Implementation wise, `rb_compile_option_struct.frozen_string_literal` is
no longer a boolean but a tri-state of `enabled/disabled/unset`.

When code is compiled with frozen string literals neither explictly enabled
or disabled, string literals are compiled with a new `putchilledstring`
instruction. This instruction is identical to `putstring` except it marks
the String with the `STR_CHILLED (FL_USER3)` and `FL_FREEZE` flags.

Chilled strings have the `FL_FREEZE` flag as to minimize the need to check
for chilled strings across the codebase, and to improve compatibility with
C extensions.

Notes:
  - `String#freeze`: clears the chilled flag.
  - `String#-@`: acts as if the string was mutable.
  - `String#+@`: acts as if the string was mutable.
  - `String#clone`: copies the chilled flag.

Co-authored-by: Jean Boussier <byroot@ruby-lang.org>

This frees FL_USER0 on both T_MODULE and T_CLASS.

Note: prior to this, FL_SINGLETON was never set on T_MODULE,
so checking for `FL_SINGLETON` without first checking that
`FL_TYPE` was `T_CLASS` was valid. That's no longer the case.

Not all `ID` have an associated string.

Fixes a SEGFAULT in ObjectSpace.dump_all spec.

This reverts commit 5f3fb4f4e397735783743fe52a7899b614bece20.

This records the called_id and klass from imemo_callcache objects in
heap dumps.

This reverts commit f6910a61122931e4193bcc0fad18d839c319b720.

We're seeing crashes in the test suite of Shopify's core monolith after
this change.