Add apply and at methods to Sized.

[milessabin]

Sized should have at and apply methods analogous to those on HList: they should yield the _n_th element of the collection iff it has at least n elements. For HLists we have,

scala> val l = 1 :: 2 :: 3 :: 4 :: HNil                                                                                                     
l: shapeless.::[Int,shapeless.::[Int,shapeless.::[Int,shapeless.::[Int,shapeless.HNil]]]] = 1 :: 2 :: 3 :: 4 :: HNil                        

scala> l(0)                                                                                                                                 
res0: Int = 1                                                                                                                               

scala> l(3)
res2: Int = 4                                                                                                                               

scala> l(4)
<console>:15: error: could not find implicit value for parameter at: shapeless.ops.hlist.At[shapeless.::[Int,shapeless.::[Int,shapeless.::[Int,shapeless.::[Int,shapeless.HNil]]]],nat_$macro$4.N]
              l(4)
               ^

The same would be useful for Sized.

[wheaties]

What should the "apply" do in this case? I can see there are already several applies within the companion object itself. Seems like it should just take some collection and use the SizedConv to return an Option. This close to the idea?

[milessabin]

@wheaties I've added a clearer description of the issue :-)

[milessabin]

Superseded by #153.