Basically a redo of https://github.com/ruby/ruby/commit/a1403fb7cbd1fe0df97c932be9814c86081783dc
but respecting the frozen string literal magic comment

Fixes [Bug #21187]

* Use FL_USER0 for ELTS_SHARED

This makes space in RString for two bits for chilled strings.

* Mark strings returned by `Symbol#to_s` as chilled

[Feature #20350]

`STR_CHILLED` now spans on two user flags. If one bit is set it
marks a chilled string literal, if it's the other it marks a
`Symbol#to_s` chilled string.

Since it's not possible, and doesn't make much sense to include
debug info when `--debug-frozen-string-literal` is set, we can't
include allocation source, but we can safely include the symbol
name in the warning message, making it much easier to find the source
of the issue.

Co-Authored-By: Étienne Barrié <etienne.barrie@gmail.com>

---------

Co-authored-by: Étienne Barrié <etienne.barrie@gmail.com>
Co-authored-by: Jean Boussier <jean.boussier@gmail.com>

The absence of either the integer or fractional part should be
allowed.

[Feature #20594]

A handy method to construct a string out of multiple chunks.

Contrary to `String#concat`, it doesn't do any encoding negociation,
and simply append the content as bytes regardless of whether this
result in a broken string or not.

It's the caller responsibility to check for `String#valid_encoding?`
in cases where it's needed.

When passed integers, only the lower byte is considered, like in
`String#setbyte`.

What a "word" is when talking about sizes is confusing because it's a
highly overloaded term. Intel, Microsoft, and GDB are just a few vendors
that have their own definition of what a "word" is. Specs that used the
"wordsize" guard actually were mostly testing for the size of the C
`long` fundamental type, so rename the guard for clarity.

Also, get the size of `long` directly from RbConfig instead of guessing
using Integer#size. Integer#size is not guaranteed to have anything to
do with the `long` type.