Added stack functions:
 - push
 - pop
 - popUnsafe
 - peek
 - peekUnsafe
 - clear
 - map
 - filter
 - choose
 - collect
 - rev
 - append
 - fold
 - foldBack
 - reduce
 - scan
 - scanBack
 - find
 - zip
 - length
 - iter
 - forall
 - distinct
 - take
 - takeWhile
 - exists

Some unit tests also.
Merge branch 'master' of https://github.com/louthy/CSharpLanguageExt.git