Proof of concept: add 'vessel.mo' as an alternative to 'vessel.dhall'

[rvanasa]

This PR is a demonstration of the serde-based value conversion feature in the Motoko VM.

In general, Motoko can now fill the role of a configuration file format in place of Dhall or JSON.

Example vessel.mo file (in place of vessel.dhall):

{
  dependencies = [ "base" ];
  compiler = null; // optional field
};

Original vessel.dhall file:

{
  dependencies = [ "base" ],
  compiler = None Text
}