Create a macro to define `VM*` types, their layout and offsets, and alias-region helpers

[fitzgen]

Not 100% sure exactly what this would look like yet, but we very mechanically repeat ourselves in multiple places:

• We define the VM* types themselves in crates/wasmtime/src/runtime/vm/vmcontext.rs
• We define their layouts and field offsets in crates/environ/src/vmoffsets.rs
• We define helpers to emit loads/stores of these types/fields with the proper alias regions in crates/cranelift/src/alias_regions.rs

And all of those need to be kept in-sync.

Probably want some kind of for_each_vm_type! macro defined in wasmtime-environ, and then use that to define the VMOffsets in wasmtime-environ, the types themselves in wasmtime::runtime::vm, and the emit helpers in wasmtime-cranelift.