Enum constraints don't seem to work

[GeorgeSapkin]

Having an enum constraint on a field causes Unknown constraint [ enum ] on column. Constraint defined as:

someField: {
    type: 'string',
    enum: [ 'typeA', 'typeB', 'typeC']
}

[davemackintosh]

I'm trying to fix as much of this as possible in my fork, hopefully @tjwebb will review and merge if we poke him enough.