Rework of #1255 : add the `call` helper function

Adds `call` helper to std.functional that makes possible to do postfix notation
lambda calls.

Based on https://github.com/D-Programming-Language/phobos/pull/1255 , adds few enhancements
as discussed with original author, @idanarye

Should now fit all comments in original discussion thread. This helper function
may be of questionable utility but it is small and does no harm ;)
Why is this line needed? `Fun` isn't used anywhere.