[PATCH] [Bug #21842] Let `rb_interned_str` return US-ASCII if possible

	[PATCH] [DOC] Update docs for rb_interned_str and related functions (#15897)

	Related to [Bug #21842].

	* rb_interned_str: document what decides whether the returned string is
	  in US-ASCII or BINARY encoding.
	* rb_interned_str_cstr: include the same description as rb_interned_str
	  for the encoding. This one was still missing the update for US-ASCII
	  and erroneously said the returned string was alwasy in BINARY encoding
	* rb_str_to_interned_str: document how the encoding of the result is
	  defined.

	Co-authored-by: Herwin <herwinw@users.noreply.github.com>

The changes are to `io.c` and `thread.c`.
I changed the API of 2 exported thread functions from `internal/thread.h` that
didn't look like they had any use in C extensions:

* rb_thread_wait_for_single_fd
* rb_thread_io_wait

I didn't change the following exported internal function because it's
used in C extensions:

* rb_thread_fd_select

I added a comment to note that this function, although internal, is used
in C extensions.

Move these macros from include/ruby/backward.h to
include/ruby/internal/attr/deprecated.h, alongside the other similar
macros.

include/ruby/internal/intern/vm.h cannot currently use them because
include/ruby/backward.h is included too late.

Implementation was removed in https://github.com/ruby/ruby/commit/a4c051b870ac7f7b3c5482baf05600e1f6751b47

rb_str_hash_cmp returns 0 if the two strings are identical and 1 if they
are different.

* Add support for `cause:` argument to `Fiber#raise` and `Thread#raise`.

The implementation behaviour is consistent with `Kernel#raise` and
`Exception#initialize` methods, allowing the `cause:` argument to be
passed to `Fiber#raise` and `Thread#raise`. This change ensures that
the `cause:` argument is handled correctly, providing a more consistent
and expected behavior when raising exceptions in fibers and threads.

[Feature #21360]

* Shared specs for Fiber/Thread/Kernel raise.

---------

Co-authored-by: Samuel Williams <samuel.williams@shopify.com>

This should be a minimal C-API needed to deal with Set objects. It
supports creating the sets, checking whether an element is the set,
adding and removing elements, iterating over the elements, clearing
a set, and returning the size of the set.

Co-authored-by: Nobuyoshi Nakada <nobu.nakada@gmail.com>