Make more code work with no_std. (#407)

* Make more code work with no_std.

no_std support is still incomplete, but this patch takes care of the
bulk of the straightforward parts.

wasmtime-wasi-c/src/translate.rs

@@ -1,5 +1,5 @@
+use core::convert::TryFrom;
 use host;
-use std::convert::TryFrom;
 use std::mem::{align_of, size_of, zeroed};
 use std::slice;
 use wasm32;