We have name fragmentation for this feature, including "shared GC",
"modular GC", and "external GC". This commit standardizes the feature
name to "modular GC" and the implementation to "GC library".

We can use the BUILDING_SHARED_GC flag to check if we're building gc_impl.h
as a shared GC or building the default GC.

Let there be rooms for each GC implementations how to handle multi
threaded situations.  They can be totally reentrant, or can have
their own mutex, or can rely on rb_thread_call_with_gvl.

In any ways the allocator (has been, but now officially is)
expected to run properly without a GVL.  This means there need be
a way for them to inform the interpreter about their allocation
failures, without relying on raising exceptions.

Let them do so by returning NULL.

So that it doesn't get included in the generated binaries for builds
that don't support loading shared GC modules

Co-Authored-By: Peter Zhu <peter@peterzhu.ca>

Co-Authored-By: Peter Zhu <peter@peterzhu.ca>

Now that we've inlined the eden_heap into the size_pool, we should
rename the size_pool to heap. So that Ruby contains multiple heaps, with
different sized objects.

The term heap as a collection of memory pages is more in memory
management nomenclature, whereas size_pool was a name chosen out of
necessity during the development of the Variable Width Allocation
features of Ruby.

The concept of size pools was introduced in order to facilitate
different sized objects (other than the default 40 bytes). They wrapped
the eden heap and the tomb heap, and some related state, and provided a
reasonably simple way of duplicating all related concerns, to provide
multiple pools that all shared the same structure but held different
objects.

Since then various changes have happend in Ruby's memory layout:

* The concept of tomb heaps has been replaced by a global free pages list,
  with each page having it's slot size reconfigured at the point when it
  is resurrected
* the eden heap has been inlined into the size pool itself, so that now
  the size pool directly controls the free_pages list, the sweeping
  page, the compaction cursor and the other state that was previously
  being managed by the eden heap.

Now that there is no need for a heap wrapper, we should refer to the
collection of pages containing Ruby objects as a heap again rather than
a size pool