Simplify the `define_query` macro
This moves a bunch of control flow out of the macro into generic functions, leaving the macro just to call the function with a new generic parameter for each query.
It may be possible to improve compile-times / icache by instantiating the generic functions only with the query key, not the query type itself, but I'm going to leave that for a follow-up PR.
Helps with https://github.com/rust-lang/rust/issues/96524.
r? `@cjgillot`