change SqlVar::type to enum

[GitMensch]

Along with enum SqlVarType SqlVar::getType() and std::vector<enum SqlVarType> param_types; - cast to/from int where absolute necessary - but keep as enum wherever possible.

This would ease debugging and for some places may provide type safety, too.

[mridoni]

This, among other things, is an holdover from the C/ocesql roots of GixSQL, there are also a few places where pointer lists are passed as int * plus an additional int for length, instead of something like const std::vector<int>& (or string). This needs to change, for all the reasons you enumerated.

[mridoni]

This is already in https://github.com/mridoni/gixsql/tree/v1.0.20dev. I am not closing the issue until the branch is merged into main.