Need to deselfify mock method arguments

[asomers]

Mockall knows that if a mocked function's return type is Self, it should replace that with the mock type. See the various constructor tests in mockall/tests. But it doesn't currently perform that transformation for argument types, unless they're self. That comes up with methods like eq, which compare two arguments of Self type. It should. For example, neither of the functions here will compile successfully:

mock! {
    #[derive(Debug)]
    pub Foo {
        fn compare(&self, other: &Foo) -> bool;
    }
    impl PartialEq for Foo {
        fn eq(&self, other: &Self) -> bool;
    }
}