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/lib.rs

@@ -33,6 +33,7 @@ pub mod fs;
 mod host;
 pub mod hostcalls;
 mod memory;
+pub mod old;
 pub mod wasi;
 pub mod wasi32;