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.
Create a `cm::String` type that holds a view of a string buffer and
optionally shares ownership of the buffer. Instances can either
borrow longer-lived storage (e.g. static storage of string literals)
or internally own a `std::string` instance. In the latter case,
share ownership with copies and substrings. Allocate a new internal
string only on operations that require mutation.
This will allow us to recover string sharing semantics that we
used to get from C++98 std::string copy-on-write implementations.
Such implementations are not allowed by C++11 so code our own in
a custom string type instead.
