Add limited CTFE

Implement compile time evaluation of a limited set of compiler-internal functions:

- `size_of`
- `min_align_of`
- `pref_align_of`

[Rendered](https://github.com/mahkoh/rfcs/blob/ctfe/active/0000-limited-ctfe.md)
All new features are going to end up landing behind a feature gate, so this part isn't really relevant.