Partial function application

[radrow]

This is an extension to #228, inspired by Scala. Function applications could be conveniently curried away, for example for convenience of higher order functions and to aid features proposed in #449 and #450.

Example syntax

function(x, y, z) = ...

f(1, _, 3)  // (y) => f(1, y, z)
f(_, 2, _) // (x, z) => f(x, 2, z)

The curried-out parameters can be numbered for more flexibility:

f($1, $2, $1)  // (a1, a2) => f(a1, a2, a1)