Refactor receiver generation in gen_call (#2064)
Refactor receiver generation in gen_call (#2064)

The process has been moved into separate functions in order to reduce
the size of gen_call itself. This doesn't introduce any logical changes.
Theodus/ponyc,cquinn/ponyc,cquinn/ponyc,kulibali/ponyc,dipinhora/ponyc,CausalityLtd/ponyc,jemc/ponyc,boemmels/ponyc,Perelandric/ponyc,dipinhora/ponyc,jemc/ponyc,Praetonus/ponyc,Perelandric/ponyc,sgebbie/ponyc,ponylang/ponyc,ponylang/ponyc,CausalityLtd/ponyc,sgebbie/ponyc,Perelandric/ponyc,sgebbie/ponyc,ponylang/ponyc,boemmels/ponyc,kulibali/ponyc,Theodus/ponyc,Perelandric/ponyc,cquinn/ponyc,Praetonus/ponyc,CausalityLtd/ponyc,dipinhora/ponyc,sgebbie/ponyc,Praetonus/ponyc,boemmels/ponyc,kulibali/ponyc,boemmels/ponyc,Theodus/ponyc,sgebbie/ponyc,jemc/ponyc,boemmels/ponyc,kulibali/ponyc,Perelandric/ponyc,Theodus/ponyc,Praetonus/ponyc,cquinn/ponyc,Theodus/ponyc