Audit existing jmespath functions for reflection usage

[jamesls]

We need to avoid checks for empty interfaces and map[string]interface{} and instead use reflection. This has been fixed in interpreter.go but functions.go needs the same treatment.

I'd also like to investigate adding autogen'd tests using structs from the compliance tests to verify we don't miss anything.

Related: #14

[tve]

I'm new to this jmespath implementation, but I was hoping that it didn't use reflection given the rather steep performance penalty. Are you basically moving away from type assertions to reflection across the board or only in very narrow cases?

[skyzyx]

This is 4 years old. What's going on here?