gojs: implements remaining link functions (#1198)

This implements the last remaining link functions using the same logic
as WASI. In doing so, this changes the signature for FS.ReadLink to be
more similar to the rest of the functions. Particularly, it stops
reading the result into a buffer.

After this, the only syscalls left to implement in gojs are chown.

Signed-off-by: Adrian Cole <adrian@tetrate.io>

internal/gojs/fs.go

@@ -664,9 +664,8 @@ func (jsfsReadlink) invoke(ctx context.Context, mod api.Module, args ...interfac
 	path := args[0].(string)
 	callback := args[1].(funcWrapper)
 
-	_ = path // TODO
-	var dst string
-	var err error = syscall.ENOSYS
+	fsc := mod.(*wasm.CallContext).Sys.FS()
+	dst, err := fsc.RootFS().Readlink(path)
 
 	return callback.invoke(ctx, mod, goos.RefJsfs, err, dst) // note: error first
 }
@@ -681,8 +680,8 @@ func (jsfsLink) invoke(ctx context.Context, mod api.Module, args ...interface{})
 	link := args[1].(string)
 	callback := args[2].(funcWrapper)
 
-	_, _ = path, link // TODO
-	var err error = syscall.ENOSYS
+	fsc := mod.(*wasm.CallContext).Sys.FS()
+	err := fsc.RootFS().Link(path, link)
 
 	return jsfsInvoke(ctx, mod, callback, err)
 }