Don't allow calling methods on wrong instantiations of generic type

[zachwolfe]

Consider the following structure Foo and its associated extend block:

Foo<|T|> :: struct { foo: T }

extend Foo<|i32|> {
    fn do_something() {
    }
}

The following code currently compiles:

Foo<|f32|>.do_something()

An error should be reported here, because the declaration of do_something is conditional on T == i32.