merge revision(s) 548086b3,3dc8cde7,e0cf80d6: [Backport #19529]

	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 548086b3 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(-)