[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>

Generating an object_id for any type other than T_OBJECT (and T_CLASS)
will inevitably allocate an IMEMO/fields objects, which isn't supported
in a NEWOBJ tracepoint.

See: https://bugs.ruby-lang.org/issues/21710#note-23

Fix up ruby/ruby#14700.

[Bug #21710]

- struct.c: `struct_alloc`

It is possible for a `NEWOBJ` tracepoint call back to write fields
into a newly allocated object before `struct_alloc` had the time
to set the `RSTRUCT_GEN_FIELDS` flags and such.

Hence we can't blindly initialize the `fields_obj` reference to `0`
we first need to check no fields were added yet.

- object.c: `rb_class_allocate_instance`

Similarly, if a `NEWOBJ` tracepoint tries to set fields on the object,
the `shape_id` must already be set, as it's required on T_OBJECT to
know where to write fields.

`NEWOBJ_OF` had to be refactored to accept a `shape_id`.

These tests use NM threads but NT is not freed for MN thread, causing it
to be reported as memory leaks in LSAN. For example:

    #1 0x62ee7bc67e99 in calloc1 gc/default/default.c:1495:12
    #2 0x62ee7bc7ba00 in rb_gc_impl_calloc gc/default/default.c:8216:5
    #3 0x62ee7bc631d1 in ruby_xcalloc_body gc.c:5221:12
    #4 0x62ee7bc5cdbc in ruby_xcalloc gc.c:5215:34
    #5 0x62ee7bdea4c6 in native_thread_alloc thread_pthread.c:2187:35
    #6 0x62ee7bdec31b in native_thread_check_and_create_shared thread_pthread_mn.c:429:39
    #7 0x62ee7bdea484 in native_thread_create_shared thread_pthread_mn.c:531:12
    #8 0x62ee7bdea1da in native_thread_create thread_pthread.c:2403:16
    #9 0x62ee7bdde2eb in thread_create_core thread.c:884:11
    #10 0x62ee7bde4466 in thread_initialize thread.c:992:16