Implement size_of directive

[feds01]

This PR implements the #size_of directive, which allows the caller to inspect
and use the size of a type within the program.

This means that we can now do things like:

foo := () => {
    ty_size := #size_of u32

    // `ty_size` is now a constant expression that can be used in the program.
    // `ty_size` has a type of `usize`
    // `ty_size` is a compile-time constant of 4.
}

The implementation itself simply captures all of these calls and expands them during TIR to then indicate that they should be in fact expanded at lowering or evaluation.

There are several restrictions to where the directive can be applied to:

• #size_of can only be called on types & expressions that look like types.

[kontheocharis]

nice and clean