Go: support for .to_s function to convert integer to string
using example from bcd.ksy (renamed to join it in this issue)

[bcd.ksy.txt](https://github.com/kaitai-io/kaitai_struct_go_runtime/files/3975753/bcd.ksy.txt)

produce good code in ruby but in GO can't have result, only error message : "an implementation is missing"