Allow assigning patterns to variables (#336)

* Change syntax tree and parser

* Add assign pattern to type inference

* Use check_expr instead of hard-coded type

* Add fcode generation for assign pattern

* Implement rename_spat for assign pattern

* Add tests

* Update CHANGELOG.md

* Update docs and changelog

* Add letpat to aeso_syntax_utils:fold

* Use Plus instead of Scoped

test/contracts/assign_pattern_to_pattern.aes

@@ -0,0 +1,4 @@
+contract AssignPatternToPattern =
+    entrypoint f() =
+        let x::(t::z = y) = [1, 2, 3]
+        (x + t)::y

test/contracts/assign_patterns.aes

@@ -0,0 +1,16 @@
+include "List.aes"
+
+contract AssignPatterns =
+
+  entrypoint test() = foo([1, 0, 2], (2, Some(3)), Some([4, 5]))
+
+  entrypoint foo(xs : list(int), p : int * option(int), some : option(list(int))) =
+    let x::(t = y::_)  = xs
+    let z::_ = t
+
+    let (a, (o = Some(b)))  = p
+
+    let Some((f = g::_)) = some
+    g + List.get(1, f)
+
+    x + y + z + a + b