Remove use of offset_of! from wasmtime-environ.

wasmtime-environ is meant to support cross compilation, so it shouldn't
have dependencies on target layout of structs. This moves the layout
back into wasmtime-execute, and adds a system of asserts for checking
that wasmtime-environ's offsets stay in sync.

lib/execute/src/lib.rs

@@ -40,6 +40,10 @@ extern crate alloc;
 #[macro_use]
 extern crate lazy_static;
 extern crate libc;
+#[cfg(test)]
+#[macro_use]
+extern crate memoffset;
+extern crate cast;
 
 mod code;
 mod execute;