Add type safe value accessor methods to product types

[fommil]

It would be good to have an accessor for products and coproducts.

Even better if there was a way to get a tuple / nested scalaz disjunction.

And for products, even better if there was a macro that allowed me to pass a Prod to a method that accepts X parameters 😄

[andyscott]

• For accessors to products, do you mean type safe indexed based accessors?

• It would be reasonable to provide two/from tuple conversions for Prod for valid tuple arities.

• What would that look like?

For the last point, consider:

def methodTakesManyParams(a: String, b: Double, c: Int): Foo = // ...
val prod = Prod[String :: Double :: Int :: TNil]("hi", 1.0, 100)

I don't think there's a way to support syntax such as

methodTakesManyParams(prod.expand)

The following may be possible, though:

prod.passTo(methodTakesManyParams)

[fommil]

prod.passTo sounds good! I don't really care about typesafe indexing (although that would be pretty cool if the index was passed as a constant!)... a .toTuple or something would be nice as a basic way to access, but maybe best not to add features that aren't needed.

[andyscott]

I broke this out into #98 and #98 for the second and third bullet points.