Add a (more constrained) API for expressing filters on columns

This is to support predicate push down, which will take advantage of these constrained filter predicates.
This will need to be reconciled with the column filters currently in the `parquet.filter` package, but I wanted to get feedback on this first.

Sample usage in java:
```
// if you lie, and column "a.b.c" isn't an int column, we won't know until runtime. 
Column<Integer> abc = column("a.b.c"); 
Column<Double> xyz = column("x.y.z");

FilterPredicate predicate =
      and(not(or(eq(abc, 7), eq(abc, 17))), gtEq(xyz, 100.0));
```

Sample usage in scala:
```
val abc = Column[Int]("a.b.c")
val xyz = Column[Double]("x.y.z")
val predicate = !((abc === 7) || (abc === 17)) && (xyz >= 100.0)
```
I would go with ```boolean <T> visit(Eq<T> eq);```
That way you at least give it a name and carry it around.