Threading macro

[SuzanneSoy]

Add a threading macro. Possible features:

• left to right or right to left: (~> x add1 sqrt) ≡ (<~ sqrt add1 x)
• implicit partial application (~> 4 sqrt (+ 10)) ; => 12
  • should the threaded value be added as the first or last argument?
• support for multiple output values (passed as multiple arguments to the next function)
• hole (~> 16 (sqrt _) (/ _ 2) (/ 1 _)) ; => 1/2