benchmarking v2 syntax is confusingly lax about return types, leading to confusion

[sam0x17]

The benchmarking v2 syntax allows any return type on benchmark function definitions. We should probably only be allowing () (blank return type) or Result<(), BenchmarkError> in scenarios where the programmer wants to use the ? operator somewhere in the benchmark function body.

After talking about this with @ggwpez we thought it would make sense to require a return type of Result<(), BenchmarkError> in scenarios where the programmer uses the ? operator within the function body, and also allow a blank () return type. Any other return types on the benchmark function definitions would trigger a compiler error. Doing things this way will make the benchmark function defintion feel more like a real function and will make people more aware that they can use ? in these contexts (and avoid the confusing situation where someone writes ? and it compiles even though what appears to be the enclosing function def does not have a Result or Option return type).

Open to any thoughts people may have, though!