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/hostcalls/mod.rs

@@ -0,0 +1,7 @@
+mod fs;
+mod misc;
+mod sock;
+
+pub use self::fs::*;
+pub use self::misc::*;
+pub use self::sock::*;