Consider refactoring Cloudevent.format()

[lance]

Based on some discussion in this pull request there are some improvements that could be made to the Cloudevent.format() function.

• Consider optimizing so that JSON.parse doesn't require several levels of indirection to get to.
• Expose a way to retrieve the data in multiple formats
• Consider moving the sanity checks into the spec instance and not in Cloudevent itself.