Remove implicit fallthrough (#2251)
Remove implicit fallthrough (#2251)

Without these returns, these switches fail to compile with gcc 7.2.0
because of -Werror=implicit-fallthrough.
jemc/ponyc,kulibali/ponyc,kulibali/ponyc,sgebbie/ponyc,sgebbie/ponyc,cquinn/ponyc,jemc/ponyc,dipinhora/ponyc,cquinn/ponyc,boemmels/ponyc,CausalityLtd/ponyc,dipinhora/ponyc,kulibali/ponyc,Theodus/ponyc,ponylang/ponyc,cquinn/ponyc,Praetonus/ponyc,CausalityLtd/ponyc,Praetonus/ponyc,ponylang/ponyc,Theodus/ponyc,boemmels/ponyc,Praetonus/ponyc,boemmels/ponyc,dipinhora/ponyc,CausalityLtd/ponyc,sgebbie/ponyc,kulibali/ponyc,Theodus/ponyc,Theodus/ponyc,sgebbie/ponyc,jemc/ponyc,boemmels/ponyc,cquinn/ponyc,boemmels/ponyc,ponylang/ponyc,sgebbie/ponyc,Praetonus/ponyc,Theodus/ponyc