Use string_view in quirk_rust_enum

Use string_view in quirk_rust_enum

quirk_rust_enum makes string copies to store in an unordered_map.
However, the original strings have an appropriate lifetime, and so no
copying is needed.  With C++17, we can rely on string_view having a
std::hash specialization, so this patch changes this code to use
string_view rather than string.