Add a type parameter to VMOffsets for pointer size (#3020)

* Add a type parameter to `VMOffsets` for pointer size

This commit adds a type parameter to `VMOffsets` representing the
pointer size to improve computations in `wasmtime-runtime` which always
use a constant value of the host's pointer size. The type parameter is
`u8` for `wasmtime-cranelift`'s use case where cross-compilation may be
involved.

* fix lightbeam

crates/lightbeam/wasmtime/src/lib.rs

@@ -175,7 +175,7 @@ struct FuncEnvironment<'module_environment> {
     module: &'module_environment Module,
 
     /// Offsets to struct fields accessed by JIT code.
-    offsets: VMOffsets,
+    offsets: VMOffsets<u8>,
 }
 
 impl<'module_environment> FuncEnvironment<'module_environment> {