Should we make BlackBox Functions heterogenous?

[kevaundray]

Currently all black box functions are defined to look the same via a FuncDefinition struct. It is however possible to have a similar structure to Directives and have each black box function define what unique types they require

[kevaundray]

One thing to consider is how the API will be used and which is more convenient and safe

[kevaundray]

See here for more context into why this could be a goo advantage.

For AND gate, the num_bits parameter is technically not needed for both inputs

[guipublic]

I answered those points in #53. Here the summary:
Non blackbox functions should be specific and have heterogeneous input
blackbox (i.e custom) functions should have homogenous input, but we should not require a num_bits for every witness.

[TomAFrench]

Resolved in #269