Fix crash in rb_gc_register_address

	[Bug #19584]

	Some C extensions pass a pointer to a global variable to
	rb_gc_register_address. However, if a GC is triggered inside of
	rb_gc_register_address, then the object could get swept since it does
	not exist on the stack.

	[Bug #19584] Register global variable address before assignment

	[Bug #19584] Register global variables before assignment

	[Bug #19584] [DOC] Tweek description of `rb_gc_register_address`

	Ensure IO.copy_stream buffer is an independent string

	Otherwise, changes to the buffer by the destination write method
	could result in data changing for supposedly independent strings.

	Fixes [Bug #21131]

	[Bug #20995] Protect `IO.popen` block from exiting by exception

	[Bug #20924] Fix reading with delimiter in wide character encodings

This reverts commit 6b73406833dd22e489114fa77c1c80c4b7af2ed0.

	Make io_fwrite safe for compaction

	[Bug #20169]

	Embedded strings are not safe for system calls without the GVL because
	compaction can cause pages to be locked causing the operation to fail
	with EFAULT. This commit changes io_fwrite to use rb_str_tmp_frozen_no_embed_acquire,
	which guarantees that the return string is not embedded.

	Don't wait in `io_binwrite_string` if not necessary. (#9792)

	---
	 io.c | 8 +++-----
	 1 file changed, 3 insertions(+), 5 deletions(-)

	`rb_io_puts` should not write zero length strings. (#7806)

	---
	 io.c                  | 40 ++++++++++++++++++++++++----------------
	 test/fiber/test_io.rb | 28 ++++++++++++++++++++++++++++
	 2 files changed, 52 insertions(+), 16 deletions(-)