Make Utf8CStr a first class citizen in C++ codebase

Utf8CStr is in many cases a better string view class than
std::string_view, because it provides "view" access to a string buffer
that is guaranteed to be null terminated. It also has the additional
benefit of being UTF-8 verified and can seemlessly cross FFI boundaries.

We would want to start use more Utf8CStr in our existing C++ codebase.
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