Refactor wasmtime-wasi crate

Changes:
* uses Rust edition 2018
* returns wasm32 errno directly rather than relying on
  wasi_common::{host, memory} modules
* wraps extraction of memory and WASI context in a macro

wasmtime-wasi/src/lib.rs

@@ -1,14 +1,3 @@
-extern crate cranelift_codegen;
-extern crate cranelift_entity;
-extern crate cranelift_wasm;
-extern crate target_lexicon;
-extern crate wasmtime_environ;
-extern crate wasmtime_jit;
-extern crate wasmtime_runtime;
-#[macro_use]
-extern crate log;
-extern crate wasi_common;
-
 mod instantiate;
 mod syscalls;