[Bug #21357] Fix crash in Hash#merge with block

	Prior to https://github.com/ruby/ruby/commit/49b306ecb9e2e9e06e0b1590bacc5f4b38169c3c
	the `optional_arg` passed from `rb_hash_update_block_i` to `tbl_update`
	was a hash value (i.e. a VALUE). After that commit it changed to an
	`update_call_args`.

	If the block sets or changes the value, `tbl_update_modify` will set the
	`arg.value` back to an actual value and we won't crash. But in the case
	where the block returns the original value we end up calling
	`RB_OBJ_WRITTEN` with the `update_call_args` which is not expected and
	may crash.

	`arg.value` appears to only be used to pass to `RB_OBJ_WRITTEN` (others
	who need the `update_call_args` get it from `arg.arg`), so I don't think
	it needs to be set to anything upfront. And `tbl_update_modify` will set
	the `arg.value` in the cases we need the write barrier.

	[Bug #21333] Prohibit hash modification inside Hash#update block

	[Bug #21331] Prohibit hash modification during stlike loop

	Remove unused retval assignments

	[Bug #21144] Win32: Use Windows time zone ID if TZ is not set

	If the TZ environment variable is not set, the time zone names
	retrieved from the system are localized for UI display and may vary
	across editions and language packs for the same time zone.
	Use the time zone IDs that are invariant across environments instead.

	[Bug #21144] Win32: Convert the time zone name to the current locale

	The Windows time zone IDs provided by Microsoft as of 24H1 are ASCII
	only all, but the API itself is not impossible to set non-ASCII key
	name.  Prefer the current locale encoding for now until we move to
	UTF-8 including environment variables and command line arguments.

	[Bug #20984] ENV.inspect should be encoding aware

	[Bug #20307] Fix `Hash#update` to make frozen copy of string keys

	Memory leak when duplicating identhash

	[Bug #20145]

	Before this commit, both copy_compare_by_id and hash_copy will create a
	copy of the ST table, so the ST table created in copy_compare_by_id will
	be leaked.

	    h = { 1 => 2 }.compare_by_identity

	    10.times do
	      1_000_000.times do
	        h.select { false }
	      end

	      puts `ps -o rss= -p #{$$}`
	    end

	Before:

	    110736
	    204352
	    300272
	    395520
	    460704
	    476736
	    542000
	    604704
	    682624
	    770528

	After:

	    15504
	    16048
	    16144
	    16256
	    16320
	    16320
	    16752
	    16752
	    16752
	    16752
	---
	 hash.c                 | 10 +++++++++-
	 test/ruby/test_hash.rb | 10 ++++++++++
	 2 files changed, 19 insertions(+), 1 deletion(-)