New CloudFlow combinators

[leafgarland]

As @dsyme suggested in mbraceproject/MBrace.Core#100, it would be good to add more functions to CloudFlow.

Here is a list of most of the F# 4.0 collection functions and which are already implemented for CloudFlow, if we could trim this to a list of functions that would be useful to add to CloudFlow then people could get started on implementing them.

Function	CloudFlow	Comment
append
averageBy	✓
average
choose	✓
chunkBySize
collect	✓
compareWith
concat
contains
countBy	✓
distinctBy	✓
distinct	✓
empty
exactlyOne
except
exists2
exists	✓
filter	✓
findBack
findIndexBack
findIndex
find	✓
fold2
foldBack2
foldBack
fold	✓
forall2
forall	✓
groupBy	✓
head
indexed
init
isEmpty	✓
item
iter2
iter	✓
iteri2
iteri
last
length	✓
map2
map3
mapFoldBack
mapFold
map	✓
mapi2
mapi
maxBy	✓
max
minBy	✓
min
nth
pairwise
permute
pick	✓
reduceBack
reduce	✓
replicate
rev
scanBack
scan
singleton
skipWhile
skip
sortByDescending	✓
sortBy	✓
sortDescending
sortWith
sort
splitInto
sumBy
sum	✓
tail
takeWhile
take	✓
truncate
tryFindBack
tryFindIndexBack
tryFindIndex
tryFind	✓
tryHead
tryItem
tryLast
tryPick	✓
unfold
where
windowed
zip3
zip

[palladin]

I'll start by removing some of them. The three main elimination criteria are

1. Sequentially
2. Global synchronization
3. On demand consumption (Pull based)
   In other words CloudFlow combinators must be Parallel and Distributed friendly and Push based by nature.

Hard-difficult combinators:

• xxx Back combinators are not Parallel friendly
• mapi (xxxi), xxxIndex, skip, scan are order sensitive and very expensive in a Parallel and Distributed setting
• xxxWhile functions require global synchronization
• zip , xxxn (like map2, zip3 etc) are Pull based
  (zip and mapxx combinators are very easy to be implemented as producers but not for intermediate step combinators. Maybe in the future we can use intermediate Buffers/Queue to implement some of them as intermediate combinators).

Easy combinators:
compareWith, concat, contains, where, average, sumBy, mapFold, max, min, sort, append, truncate, chunkBySize

That's all for now!