StringKey: improve lookups with a string-like key

The following discussion from !1762 (merged) should be addressed:

@graven:

That reminds me: now that C++20 has heterogeneous lookup for std::unordered_map we can improve on what StringKey was intended for -- namely speed up lookups with a string-like key. See eg https://ebadblog.com/looking-up-a-c++-hash-table-with-a-pre-known-hash