add the chainable macro

Chainable masks the named function with a macro that auto-calls chain.
I found I had blocks like:

(defmacro $ (target &rest calls)
  (apply macros.chain (cons (macros.call "$" target) calls)))

(defmacro moment (target &rest calls)
  (apply macros.chain (cons (macros.call 'moment target) calls)))

At the top of my files, so this DRYs them up.  Now it's just
`(chainable $) (chainable moment)`