Added `fun` draft

R is a functional programming language but there is no terse syntax for
lambdas. There are several packages which implement a terse lambda type
but they aren’t what I’m looking for. In particular, lambda.r, while
pretty good, doesn’t actually offer terseness.

Other packages introduce slightly weird syntax. I think my approach is
more intuitive. For now, the name of the function is `fun` but I will
replace this by plain `.` in the future. That may seem cryptic at first
but I argue that first-class functions are so central to R that they
benefit from having the tersest possible syntax.
Merge branch 'fun'