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

docs/sophia_features.md

@@ -471,6 +471,17 @@ function
   get_left(Both(x, _)) = Some(x)
 ```
 
+Sophia also supports the assignment of patterns to variables:
+```sophia
+function f(x) = switch(x)
+  h1::(t = h2::_) => (h1 + h2)::t  // same as `h1::h2::k => (h1 + h2)::h2::k`
+  _ => x
+
+function g(p : int * option(int)) : int =
+  let (a, (o = Some(b))) = p  // o is equal to Pair.snd(p)
+  b
+```
+
 *NOTE: Data types cannot currently be recursive.*
 
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