Sync with wasmtime and update public interface

Internal modules `memory` and `host` can indeed be internal hidden
behind public-private visibility as `wasmtime-wasi` has already
been updated not to use the said modules (see
CraneStation/wasmtime#298).

src/lib.rs

@@ -28,10 +28,9 @@ mod hostcalls_impl;
 mod sys;
 #[macro_use]
 mod macros;
-
-pub mod host;
+mod host;
 pub mod hostcalls;
-pub mod memory;
+mod memory;
 pub mod wasm32;
 
 pub use ctx::{WasiCtx, WasiCtxBuilder};