GH-196 pattern matching lhs (#210)

* Allow block with separate type signature and definition of a function

For instance,
```
function
  add : (int, int) => int
  add(x, y) = x + y
```

cc #196

* Allow pattern matching in left-hand sides

* Changelog

* Fix type spec

* partial case-on-constructor

* Changelog for pattern-matching lets

test/contracts/factorial.aes

@@ -11,7 +11,7 @@ contract Factorial =
 
   stateful entrypoint set_worker(worker) = put(state{worker = worker})
 
-  entrypoint fac(x : int) : int =
-    if(x == 0) 1
-    else x * state.worker.fac(x - 1)
-
+  entrypoint
+    fac : int => int
+    fac(0) = 1
+    fac(x) = x * state.worker.fac(x - 1)