Add tuple syntax

[danicheg]

This PR brings 4 handy methods for tuples (for Scala 2 only):

val foo = (1, 2, 4).head // 1
val bar = (1, 2, 4).init // (1, 2)
val baz = (1, 2, 4).last // 4
val qux = (1, 2, 4).tail // (2,4)

That functionality there is in shapeless https://github.com/milessabin/shapeless/blob/main/core/src/main/scala/shapeless/syntax/std/tuples.scala#L30. However, bringing shapeless as a dependency just for tuple syntax is overkill, I think. What's more, implementation in mouse is plain and not used type-level magic, so it will not affect compile time at all.

[benhutchison]

Thats cool! I always liked init and last on List.

I have one future-oriented query. Scala 3 is released and gradually the community will migrate to it. The implementation of init and last in 3+ would be more elegant and not subject to arity limit. Assuming they don't offer these methods in core lib, are we stuck with clunky 2.x approach forever more for compat reasons..? Or can a future 3.x+ only version of mouse offer these operations built on top of "HList tuple" model?

[danicheg]

@benhutchison thanks for the review! I have pondered about init and last methods on tuples in Scala 3 and eventually make a PR to core lib. So if core lib will have those methods, it makes sense to have the syntax in mouse only for Scala 2. So this PR will be waiting for a decision on scala/scala3#13735.

[danicheg]

@benhutchison PR in dotty was merged, so I've changed tuple syntax to be available only for Scala 2.

[benhutchison]

@danicheg apology for slows, I like the plan to make avail in 2.12 only and your PR to core 👏

[danicheg]

@benhutchison sorry for yet another ping. I have ended up working on this and would be extremely glad for your review.