revert capnpc-go/capnpc-go.go to 4243b225e0141e4b7921a8dd4dcb179f545908b0
Add "customtype" annotation for :Data
Fix JSON for void field slots

If the name of a Void field wasn't "void" generated code would be uncompilable
finish reverting capnpc-go/capnpc-go.go so that JSON_enabled = true works again