[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

This reverts commit bb180b87b43c45e17ff49735a26d7a188d5c8396, which
caused "malloc during GC" error on wasm.

	[Bug #21331] Prohibit modification during stlike loop

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 #20984] ENV.inspect should be encoding aware

	[Bug #20984] Fix test with locale encoding

The inspect format was intentionally changed as an outcome of
[Bug #20433] [ruby-core:118668], but some documentation update
was missing, as [Bug #20962] pointed out. Update some output
examples that clearly use Hash#inspect.