Support for limited compile-time arithmetic on Val types

[iamed2]

After this PR:

@inferred Val{2} + Val{1} == Val{3}

I was doing inlineable type-stable recursion in AxisArrays but wanted to be able to specify an early terminator. I started with just decrement but decided to do +, -, and * in case other use cases came up.

[vtjnash]

no, but I should have Inference able to do this soon enough :)

[andyferris]

Very funny.

I got a PR open for a Val(x) constructor so that this might instead become

Val(2) + Val(1) == Val(3)

I've also had a long-standing plan to make a StaticNumbers package to live alongside StaticArrays that does this stuff (e.g. SInt as "static integer").

But by far the best plan is as Jameson says - make inference do this propagation automatically :)