appspec/client support

[joe-p]

When interacting with an ABI method from another contract, one must manually define the name, argument types, and return type. It would be nice if there was a way to use either the appspec or typed client inside TEALScript contracts.

I think the best approach to this would be a utility that generates a .d.ts file with the method signatures that can be used in the contract and an embedded appspec that can be parsed by the compiler.