Add return type into the ABI

[kevaundray]

Problem

The ABI currenlty only specifies parameters and not the return types from the functions. This is not a big problem as we use the "return" parameter to denote the output. Assuming there are no other parameters named "return" this is safe.

Solution

We can add another field in the ABI Parameter field called "return" and put the return type there instead.

Alternatives considered

(Describe any alternative solutions you have considered.)

Additional context

(If applicable.)

[TomAFrench]

This is a duplicate of #678. Closing this issue as the other has more detail.