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

[ydnar]

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<T>, 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.