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