Corrected impl lookup to search whole scope and fixed a bug with tuple/unit structs at end of scope

[Jonesey13]

This PR fixes two issues. Firstly there is an issue where racer could panic when completing on a tuple or unit struct at the end of a scope:

mod sub {
    impl Foo {
        pub fn two(&self) -> usize {2} 
    }
    pub struct Foo(bool);
}

fn main() { // l16
    let t = sub::Foo(true);
    t.t
}

Originally, if one tried to complete on t.t, racer would panic since it would try to find fields on the struct Foo, look ahead for an open/close brace and panic when it meets the } at the end of the Sub module. This is because of an u32 underflow in scopes::find_close which has now been fixed.

Secondly, the completion at t.t above would still fail since racer would start searching for implementations from the F in pub struct Foo(bool) rather than from the start of the module scope. This has now been fixed and a new test has been added to cover this case.

[phildawes]

This is great, thanks @Jonesey13