Remove unused "setup" functions in interpreter

[d-torrance]

This is based on a few commits that I'd originally proposed in #3419.

We remove some of the setup family of functions in common.d (which are used for associating a symbol with its function) that weren't being used anywhere:

• All of them that accept a Word argument.
• setup(SymbolClosure, unop, unop) (for unary operators that are both prefix and postfix, of which we have none)
• setupfun(string, unop)

We also remove setup(SymbolClosure, unop) since it was identical to setupop.

[mahrud]

I probably won't have enough time to do this review for a few weeks, but then I'd like to take a better look at how and where setup-like functions are used, mainly because before removing them I want to make sure we can't make better simplifications elsewhere using them.