Disable returning types from functions entirely

[pzipper]

Currently, type technically can be used as a return type for a function, but type values are not supported by the Cranelift backend.

const MyFunc = func(): type {
    return i32;
};

Which causes the compiler to output:

thread 'main' panicked at 'all possible types in Amp should be supported by Cranelift', src\clif\mod.rs:243:18

Instead, we should report a diagnostic explaining that types cannot be returned from functions.