Is your feature request related to a problem or challenge?

While I was working on #18337 I saw a lot of
string copying happening when calculating qualified names and types and fields.

For example, in Expr::to_field there are clones all over the place:

There are also several places where the field names are copied at least twice

Describe the solution you'd like

Stop copying so much

Describe alternatives you've considered

No response

Additional context

No response