We are going to significantly extend py-strings related functionality soon
- to the point where amount of strings related code will be
approximately the same compared to the amount of all other
python-related code inside golang module.

-> First move everything related to py strings to dedicated
_golang_str.pyx as a preparatory step.

Keep that new file included from _golang.pyx instead of being real new
module, because we want strings functionality to be provided by golang
main namespace itself, and to ease internal code interdependencies.

Plain code movement.

/reviewed-by @jerome
/reviewed-at nexedi/pygolang!18