Refactor $utils.map functions

[marklawlor]

This is an untested PR to fix #70

I'm working on an unrelated project today, but I'll test this over the weekend

[marklawlor]

So the test's are failing as they are testing with an object and not a JavaMap. Quick glance at the code base, there are number of other places where an object is returned instead of JavaMap.

@cbaron @lightsofapollo I'm assuming all the vtl operations should be using a JavaMap? Or am I missing something. I'm not sure what the fallout would be if everything was converted to a JavaMap

[lightsofapollo]

@marklawlor afaik returning an object is a bug and everything should be a java map (otherwise operations that work in aws appsync context will fail here)