Slight gen revert

Created by: cbusbey

In practice, this seems to be the more code friendly impl:

//NewAccount returns a new AccountField initialized with val
func NewAccount(val string) AccountField {
    return AccountField{quickfix.FIXString(val)}
}

vs.

//NewAccount returns a new AccountField initialized with val
func NewAccount(val quickfix.FIXString) AccountField {
    return AccountField{val}
}

The latter forces some type gymnastics in most common cases where we have a string var to pass into NewAccount.