Unify output field node type when selecting by field type

[alonsodomin]

At the moment, when having a parsed CRON expression and selecting one of its fields by it's CronField type, the return type is a path-dependent type result of the machinery used to do this selection in a type-safe way:

scala> cron.field[CronField.Second]
res0: cron4s.expr.FieldSelector.SecondsFromCronExpr.Out[CronField.Second] = 8

This in contrast to the return type obtained when selecting by the field accessor:

scala> cron.seconds
res1: cron4s.expr.SecondsNode = 8

In the first case, the path-dependent type is an useless artifact since right after obtaining the reference to the field, we can't do much more with other than printing it's expression.

The type of field[CronField] should unify to the actual type of the node, otherwise that operation has very little sense to exist.