Rename visitor member to reflect actual purpose
Rename visitor member to reflect actual purpose

As @vmagamedov requested, changing `_RecordFieldsValidator._fields` to
`_RecordFieldsValidator._field_types`, as the latter reflects stored
data and purpose better.

vmagamedov/hiku,vmagamedov/hiku,vmagamedov/hiku,vmagamedov/hiku