embedded stdlib

This PR embeds the stdlib into the dagger binary itself for convenience.
Long term we will want to source the stdlib directly from git.

- Updated go to 1.16 to use the new built-in embedding functionality
- The `stdlib` go package now contains an embed of the stdlib

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

go.mod

@@ -1,6 +1,6 @@
 module dagger.cloud/go
 
-go 1.13
+go 1.16
 
 require (
 	cuelang.org/go v0.3.0-beta.4