Fix crash in tracing object allocations

	ObjectSpace.trace_object_allocations_start could crash since it adds a
	TracePoint for when objects are freed. However, TracePoint could crash
	since it modifies st tables while inside the GC that is trying to free
	the object. This could cause a memory allocation to happen which would
	crash if it triggers another GC.

	See a crash log: http://ci.rvm.jp/results/trunk@ruby-sp1/4373707
	---
	 ext/objspace/depend           | 1 +
	 ext/objspace/object_tracing.c | 7 +++++++
	 gc.h                          | 4 ++--
	 3 files changed, 10 insertions(+), 2 deletions(-)

This commit adds a `capacity` field to shapes, and adds shape
transitions whenever an object's capacity changes. Objects which are
allocated out of a bigger size pool will also make a transition from the
root shape to the shape with the correct capacity for their size pool
when they are allocated.

This commit will allow us to remove numiv from objects completely, and
will also mean we can guarantee that if two objects share shapes, their
IVs are in the same positions (an embedded and extended object cannot
share shapes). This will enable us to implement ivar sets in YJIT using
object shapes.

Co-Authored-By: Aaron Patterson <tenderlove@ruby-lang.org>

[Misc #18891]

... to allow class.c to use the function

Linking printf family functions makes mjit objects to link
unnecessary code.

This commit introduces Ractor mechanism to run Ruby program in
parallel. See doc/ractor.md for more details about Ractor.
See ticket [Feature #17100] to see the implementation details
and discussions.

[Feature #17100]

This commit does not complete the implementation. You can find
many bugs on using Ractor. Also the specification will be changed
so that this feature is experimental. You will see a warning when
you make the first Ractor with `Ractor.new`.

I hope this feature can help programmers from thread-safety issues.