Add optional typing

[lausek]

Todo

• [ ] allow emitting more than one error when lowering the hir
• [ ] introduce a feature-flag for typing support
• [ ] give each Expr an optional type
• [ ] check that there is no type violation when lowering
• [ ] respect type changes when casting

[lausek]

This should be implemented as a separate crate on top of the HIR to keep lovm2 slim