String: Add a custom string type
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.
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