String: Add str_if_stable() as a const alternative to str()
The `str()` method must be non-const because it may need to internally mutate the representation of the string in order to have an owned `std::string` instance holding the exact string (not a superstring). This is inconvenient in contexts where we can ensure that no mutation is needed to get a `std::string const&`. Add a `str_if_stable() const` method that returns `std::string const*` so we can return `nullptr` if if mutation would be necessary to get a `std::string const&`. Add supporting `is_stable() const` and `stabilize()` methods to check and enforce stable availability of `std::string const&`. These can be used to create `String const` instances from which we can still get a `std::string const&` via `*str_if_stable()` by maintaining the stability invariant at runtime.