Repair various cases where values of distinct types were being operated
upon (e.g., `&int` added to `int`).
Add impls of the comparison operators for fixed-length arrays of lengths 0...32 and repair various cases where slices and fixed-length arrays were being compared.
Implement new operator dispatch semantics.

Key points are:
1. `a + b` maps directly to `Add<A,B>`, where `A` and `B` are the types of `a` and `b`.
2. Indexing and slicing autoderefs consistently.