We mostly use classical C strings, but the memory management around them
can get confusing and error prone. Let's use std::string for the cases
where we need to return a newly allocated string.

This allows us to separate accidentally unused, from explicitly unused
parameters, which allows us to turn on such checks in the compiler.

The generally recommended way is to include it from source files, not
headers. We had a mix of both. Let's try to be consistent and follow the
recommended way.

It's either not used, or no longer relevant.

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