Warn or error on path statements

[brson]

This is pretty useless and error prone:

fn do_something_important() {
}

fn main() {
  do_something_important;
}

It's just mentioning a function, when the user probably wanted to call it. Note that it's not useless when leaving off the semi.

[brson]

Looks like compile-fail/does-nothing.rs is testing this or something similar

[catamorphism]

does_nothing tests the case where do_something_important is unresolved. This is a different situation.