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

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - `Set` stdlib
 - `Option.force_msg`
 - Loading namespaces into the current scope (e.g. `using Pair`)
+- Assign patterns to variables (e.g. `let x::(t = y::_) = [1, 2, 3, 4]` where `t == [2, 3, 4]`)
 ### Changed
 ### Removed