Add withFilter and filter to XorT

[mkotsbak]

This will allow to use if statements in for-comprehensions. See similar issue for OptionT: #981

[ceedubs]

I think that OptionT is a bit different than XorT in this context. If you filter out an item from an Option then you can naturally return None. What do you return in the case of Xor/XorT for an empty value? Xor used to require a Monoid instance for the left type and use the empty value from that Monoid, but it was removed in #276 as it was pretty ad-hoc and arbitrary. For example what should the following return?

val x: String Xor Int = Right(3)
val y = x.filter(_ % 2 == 0)

With a Monoid empty it would return Left(""). To me this seems pretty arbitrary and probably unhelpful. I think it's better to do something like:

val x: String Xor Int = Right(3)
val y = x.ensure("oops it was odd!")(_ % 2 == 0)

Note that I'm using Xor in these examples but I think it all applies equally well to XorT.

[mkotsbak]

Agree, this is not solvable.