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)
I meant that this should be behind a feature gate until real CTFE has been implemented (if ever). Real CTFE would immediately replace this, but because this is not 100% compatible you would have to continue to maintain this if it's not marked unstable in same way.