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/Cargo.toml

@@ -7,6 +7,7 @@ categories = ["wasm"]
 repository = "https://github.com/CraneStation/wasmtime"
 license = "Apache-2.0 WITH LLVM-exception"
 readme = "README.md"
+edition = "2018"
 
 [dependencies]
 wasmtime-runtime = { path = "../wasmtime-runtime" }