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.