RDoc was making every usage of the word "GC" link to the page for GC
(which is the same page).

RDoc parses the last arrow in the call-seq as the arrow for the return
type. It was getting confused over the arrow in the hash.

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>

In order to reliably test compaction we need to be able to move objects
between size pools.

In order for this to happen there must be pages in a size pool into
which we can allocate.

The existing implementation of `double_heap` only doubled the existing
number of pages in the heap, so if a size pool had a low number of pages
(or 0) it's not guaranteed that enough space will be created to move
objects into that size pool.

This commit deprecates the `double_heap` option and replaces it with
`expand_heap` instead.

expand heap will expand each heap by enough pages to hold a number of
slots defined by `GC_HEAP_INIT_SLOTS` or by `heap->total_pags` whichever
is larger.

If both `double_heap` and `expand_heap` are present, a deprecation
warning will be shown for `double_heap` and the `expand_heap` behaviour
will take precedence

Given that this is an API intended for debugging and testing GC
compaction I'm not concerned about the extra memory usage or time taken
to create the pages. However, for completeness:

Running the following `test.rb` and using `time` on my Macbook Pro shows
the following memory usage and time impact:

pp "RSS (kb): #{`ps -o rss #{Process.pid}`.lines.last.to_i}"
GC.verify_compaction_references(double_heap: true, toward: :empty)
pp "RSS (kb): #{`ps -o rss #{Process.pid}`.lines.last.to_i}"

❯ time make run
./miniruby -I./lib -I. -I.ext/common  -r./arm64-darwin21-fake  ./test.rb
"RSS (kb): 24000"
<internal:gc>:251: warning: double_heap is deprecated and will be removed
"RSS (kb): 25232"

________________________________________________________
Executed in  124.37 millis    fish           external
   usr time   82.22 millis    0.09 millis   82.12 millis
   sys time   28.76 millis    2.61 millis   26.15 millis

❯ time make run
./miniruby -I./lib -I. -I.ext/common  -r./arm64-darwin21-fake  ./test.rb
"RSS (kb): 24000"
"RSS (kb): 49040"

________________________________________________________
Executed in  150.13 millis    fish           external
   usr time  103.32 millis    0.10 millis  103.22 millis
   sys time   35.73 millis    2.59 millis   33.14 millis

Define `GC.verify_compaction_references` as a built-in ruby method,
according to GC compaction support via `GC::OPTS`.

These methods are removed from gc.rb and added to gc.c:

- GC.compact
- GC.auto_compact
- GC.auto_compact=
- GC.latest_compact_info
- GC.verify_compaction_references

This is a prefactor to allow setting these methods to
`rb_f_notimplement` in a followup commit.

GC.stat_heap will return stats for memory heaps. This is
used for the Variable Width Allocation feature.

Adds documentation for keys `time`, `compact_count`,
`read_barrier_faults`, `total_moved_objects`.