Add support for wasi_snapshot_preview1. (#592)

* Add support for wasi_snapshot_preview1.

This adds support for the new ABI, while preserving compatibility
support for the old ABI.

* Fix compilation on platforms where nlink_t isn't 64-bit.

* rustfmt

* Fix Windows build errors.

crates/wasi-common/src/old/snapshot_0/mod.rs

@@ -0,0 +1,16 @@
+mod ctx;
+mod error;
+mod fdentry;
+mod helpers;
+mod host;
+pub mod hostcalls;
+mod hostcalls_impl;
+mod memory;
+mod sys;
+pub mod wasi;
+pub mod wasi32;
+
+pub use ctx::{WasiCtx, WasiCtxBuilder};
+
+pub type Error = error::Error;
+pub type Result<T> = std::result::Result<T, Error>;