Add return value accessor to assigner

[aleasims]

After successful evaluation assigner has a return value of circuit function, which now can be printed to stdout (see print_format). However we cannot access this value from API now. It will be nice for users and really helpful for testing assigner.

I suggest we add public method get_return_value:

std::vector<typename BlueprintFieldType::integral_type> get_return_value();

• If the return value is primitive (integer, field), this returns vector of a single element - the value itself;
• Using BlueprintFieldType::integral_type here allows to return single value for non-native fields (at least as I understood it);
• If the return value is LLVM vector of curve, this returns vector of values of corresponding size;
• Other cases (pointers, structs and etc.) are not defined yet.

If the function is called not after successful evaluation, result must be undefined (probably empty).

[aleasims]

@akokoshn @CblPOK-git
Is std::vector<typename BlueprintFieldType::integral_type> fine as a return value type?

[CblPOK-git]

We create all issues related to submodules in the zkllvm repository if I'm right

[CblPOK-git]

BlueprintFieldType::integral_type

It is basically uint256, works fine with pallas, but bls12381 has bit length of 381, need to use extended_integral_type which is something like uint4096.

[aleasims]

For now only a partial implementation, which does not handle complicated cases with aggregated return types (just like it is done in printing output).

The rest of implementation will be covered in

• Implement return values for aggregated types #218