Add support for specifying generic arg in path segment

[Y-Nak]

Currently, it's not allowed to specify type args for each path segment. This problem makes it impossible to call methods in some cases. e.g.,

struct Foo<T> {
    t: T
}

impl<T> Foo<T> {
    fn method() -> i32 {
        1
    }
}

fn foo() {
    // Deciding the `Foo` type is not possible without a type argument for `Foo`.
    let x = Foo::method()

    // We need this! 
    let x = Foo<i32>::method()
}