opal/core: add Base64 decoding implementation.
opal/core: add Base64 decoding implementation.

Opal's built-in base64 implementation is a bit broken.  Add a helper to
decode base64.

Signed-off-by: brian m. carlson <738bdd359be778fee9f0fc4e2934ad72f436ceda@crustytoothpaste.net>

bk2204/daniel-ruby,bk2204/daniel-ruby