ObjectSpace::WeakMap: clean inverse reference when an entry is
	 re-assigned

	[Bug #19531]

	```ruby
	wmap[1] = "A"
	wmap[1] = "B"
	```

	In the example above, we need to remove the `"A" => 1` inverse reference
	so that when `"A"` is GCed the `1` key isn't deleted.
	---
	 test/ruby/test_weakmap.rb | 17 +++++++++
	 weakmap.c                 | 91 ++++++++++++++++++++++++++++++++++++++---------
	 2 files changed, 91 insertions(+), 17 deletions(-)

	Add guard to compaction test in WeakMap

	Some platforms don't support compaction, so we should skip this test.
	---
	 test/ruby/test_weakmap.rb | 2 ++
	 1 file changed, 2 insertions(+)

	ObjectSpace::WeakMap: fix compaction support

	[Bug #19529]

	`rb_gc_update_tbl_refs` can't be used on `w->obj2wmap` because it's
	not a `VALUE -> VALUE` table, but a `VALUE -> VALUE *` table, so
	we need some dedicated iterator.
	---
	 test/ruby/test_weakmap.rb |  8 ++++++++
	 weakmap.c                 | 37 ++++++++++++++++++++++++++++++++++++-
	 2 files changed, 44 insertions(+), 1 deletion(-)

	Fix crash during compaction

	[Bug #19529]

	The fix for [Bug #19529] in commit 548086b contained a bug that crashes
	on the following script:

	```
	wm = ObjectSpace::WeakMap.new
	obj = Object.new
	100.times do
	  wm[Object.new] = obj
	  GC.start
	end
	GC.compact
	```
	---
	 test/ruby/test_weakmap.rb | 10 ++++++++++
	 weakmap.c                 |  2 +-
	 2 files changed, 11 insertions(+), 1 deletion(-)

	Fix incorrect size of WeakMap buffer

	In wmap_final_func, j is the number of elements + 1 (since j also
	includes the length at the 0th index), so we should resize the buffer
	to size j and the new length is j - 1.
	---
	 weakmap.c | 4 ++--
	 1 file changed, 2 insertions(+), 2 deletions(-)

	[Bug #19398] Memory leak in WeakMap

	There's a memory leak in ObjectSpace::WeakMap due to not freeing
	the `struct weakmap`. It can be seen in the following script:

	```
	100.times do
	  10000.times do
	    ObjectSpace::WeakMap.new
	  end

	  # Output the Resident Set Size (memory usage, in KB) of the current Ruby process
	  puts `ps -o rss= -p #{$$}`
	end
	```
	---
	 gc.c                      | 1 +
	 test/ruby/test_weakmap.rb | 9 +++++++++
	 2 files changed, 10 insertions(+)

[feature #16035]

This goes one step farther than what nobu did in [feature #13498]

With this patch, special objects such as static symbols, integers, etc can be used as either key or values inside WeakMap. They simply don't have a finalizer defined on them.

This is useful if you need to deduplicate value objects

* gc.c (wmap_aset): bypass check for frozen and allow frozen
  object in WeakMap.  [Bug #13498]