wit, wit/bindgen: support abstract "pointer" type, size, and alignment

In order to support `wasm64`, with 64-bit wide pointers, we need to relax the constraint in the Component Model and Canonical ABI that pointer types fit in 32 bits.

Ideally the generated Go code for a component can handle pointer widths of either 32 or 64 bits without baking assumptions into the precise memory layout of structures into the lifting and lowering code.

### Tasks

- [ ] Reconcile this with proposed `wasm64` support in the Component Model spec.
- [ ] Introduce "integer pointer" type (e.g. `uintptr`) with size/alignment that varies based on arch. For instance, when flattening a `variant` type with a `string` or `list`, the flattened pointers resolve to a `uintptr` in Go.
- [ ] Thread this flexible width concept through the ABI, lifting, and lowering code in `wit` and `wit/bindgen`.