[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]>