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.