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

src/aeso_syntax.erl

@@ -40,6 +40,8 @@
               | {type_decl, ann(), id(), [tvar()]}
               | {type_def, ann(), id(), [tvar()], typedef()}
               | {fun_decl, ann(), id(), type()}
+              | {fun_clauses, ann(), id(), type(), [letbind()]}
+              | {block, ann(), [decl()]}
               | letbind().
 
 -type compiler_version() :: [non_neg_integer()].
@@ -48,7 +50,7 @@
 
 -type letbind()
     :: {letval, ann(), pat(), expr()}
-     | {letfun, ann(), id(), [arg()], type(), expr()}.
+     | {letfun, ann(), id(), [pat()], type(), expr()}.
 
 -type arg() :: {arg, ann(), id(), type()}.