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