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

CHANGELOG.md

@@ -6,6 +6,26 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 ### Added
+- Allow separate entrypoint/function type signature and definition, and pattern
+  matching in left-hand sides:
+  ```
+    function
+      length : list('a) => int
+      length([])      = 0
+      length(x :: xs) = 1 + length(xs)
+  ```
+- Allow pattern matching in list comprehension generators (filtering out match
+  failures):
+  ```
+    function somes(xs : list(option('a))) : list('a) =
+      [ x | Some(x) <- xs ]
+  ```
+- Allow pattern matching in let-bindings (aborting on match failures):
+  ```
+    function test(m : map(int, int)) =
+        let Some(x) = Map.lookup(m, 0)
+        x
+  ```
 ### Changed
 - FATE code generator improvements.
 ### Removed