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/src/lib.rs

@@ -1,6 +1,7 @@
 #![allow(improper_ctypes)]
 
 mod instantiate;
+pub mod old;
 mod syscalls;
 
 pub use instantiate::{create_wasi_instance, instantiate_wasi, instantiate_wasi_with_context};