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/syscalls.rs

@@ -99,7 +99,7 @@ fn get_memory(vmctx: &mut VMContext) -> Result<&mut [u8], wasi::__wasi_errno_t>
                     "no export named \"memory\", or the export isn't a mem: {:?}",
                     x
                 );
-                Err(wasi::__WASI_EINVAL)
+                Err(wasi::__WASI_ERRNO_INVAL)
             }
         }
     }