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/Cargo.toml

@@ -19,12 +19,16 @@ region = "1.0.0"
 lazy_static = "1.2.0"
 libc = { version = "0.2.44", default-features = false }
 errno = "0.2.4"
+cast = { version = "0.2.2", default-features = false }
 
 [build-dependencies]
 cmake = "0.1.35"
 bindgen = "0.44.0"
 regex = "1.0.6"
 
+[dev-dependencies]
+memoffset = "0.2.1"
+
 [features]
 default = ["std"]
 std = ["cranelift-codegen/std", "cranelift-wasm/std"]