This only affects current CKAN master (future 2.10) but flagging in advance

After ckan/ckan#6048 was merged, trying to use a schema that uses builtin functions like unicode (py2) or str fails with:

E           TypeError: str cannot be used as validator because it is not a user-defined function

The example schema that this extension ships with, and some of the presets use some of these builtin validators/converters so we should replace them with the alternatives currently used in CKAN core (ie unicode_safe).

For previous versions of CKAN I think ckanext-scheming should provide its own copies of the validators, or include them in ckantoolkit.