Support @ patterns as in Haskell
Update 'at patterns' to use the as notation per mailing list discussion
Merge pull request #167 from A1kmm/Support-at-patterns

Support @ patterns as in Haskell
Only produce elmo files. The aim is to use a layout for object files similar to Go, which has thought very hard about how to make compilation really fast.
Merge branch 'master' of https://github.com/evancz/Elm
Merge branch 'master' into dev

Conflicts:
	compiler/Model/Ast.hs
	compiler/Types/Constrain.hs
Fix parsing bug in `as` patterns that would fully commit if any spaces were seen.

Also add `as` variables to reported variables in patterns.