* cannon: Remove memory.SetUint32

Remove uint32 word stores from the `mipsevm.memory` interface. `SetUint32` is inflexible
due to its word-alignment constraints. This prevents tests for 32 and 64-bit VMs from
using the same program counter values when writing instructions to memory.

Instead, tests should use the new `testutil.StoreInstruction` utility function to write instructions to any naturally aligned memory location.

* use arch.Word csats in go-ffi