Ruby

Implement chilled strings

[Feature #20205]

As a path toward enabling frozen string literals by default in the future,
this commit introduce "chilled strings". From a user perspective chilled
strings pretend to be frozen, but on the first attempt to mutate them,
they lose their frozen status and emit a warning rather than to raise a
FrozenError.

Implementation wise, rb_compile_option_struct.frozen_string_literal is
no longer a boolean but a tri-state of enabled/disabled/unset.

When code is compiled with frozen string literals neither explictly enabled
or disabled, string literals are compiled with a new putchilledstring
instruction. This instruction is identical to putstring except it marks
the String with the STR_CHILLED (FL_USER3) and FL_FREEZE flags.

Chilled strings have the FL_FREEZE flag as to minimize the need to check
for chilled strings across the codebase, and to improve compatibility with
C extensions.

Notes:

• String#freeze: clears the chilled flag.
• String#-@: acts as if the string was mutable.
• String#+@: acts as if the string was mutable.
• String#clone: copies the chilled flag.

Co-authored-by: Jean Boussier [email protected]