Add missing operators

[RalfJung]

Rust has a bunch of operators that we do not support yet:

• [x] BitOr, BitXor (on integers and Booleans)
• [x] Shl, Shr
• [x] Cmp
• [x] checked operators (in MIR encoded via a separate Rvalue::CheckedBinaryOp) -- AFAIK this is only allowed for Add, Sub, Mul; they should become separate BinOp for us
• [x] some intrinsics that are lowered to binops: AddUnchecked, SubUnchecked, MulUnchecked, ShlUnchecked, ShrUnchecked
• [x] UnOp::Not, which we support for Booleans but not for integers
• [x] Comparison operators on bool (here it seems best to not add anything new to MiniRust, and teach minimize to translate this to operations on integers)
• [ ] Comparison operators on pointers (probably also best done via casts)