On places that `pub` would be "reasonable" but incorrect, provide better errors

[estebank]

When trying to incorrectly specify a pub fn in a trait, the error message just complains about the incorrect token:

trait T {
    pub fn foo(&self);
}

Ideally the compiler would explain that individual trait methods' visibility can't be modified, only the trait itself, and continue parsing the AST as if it were correct, possibly suggesting the following, if appropriate:

pub trait T {
    fn foo(&self);
}