Make nested datatypes into errors

I was hoping to add nested datatypes into the language, but it turns out
doing so is quite tricky and not all that useful with Roc's current
compilation model. Basically every implementation strategy I could think
of ended up requiring a uniform representation for the data layout
(or some ugly workaround). Furhermore it increased the complexity of the
checker/mono IR generator a little bit - basically, we must always pass
around the alias definitions of nested datatypes and instantiate them
at usage sites, rather than being able to unroll aliases as we currently
do during canonicalization.

So, especially because we don't support polymorphic recursion anyway, I
think it may be better to simply disallow any kind of nested datatypes
in the language. In any case, Stephanie Weirich [seems to think nested
datatypes are not needed](https://www.cis.upenn.edu/~plclub/blog/2020-12-04-nested-datatypes/).

Closes #2293

compiler/parse/src/ast.rs

@@ -232,6 +232,16 @@ pub struct AliasHeader<'a> {
     pub vars: &'a [Loc<Pattern<'a>>],
 }
 
+impl<'a> AliasHeader<'a> {
+    pub fn region(&self) -> Region {
+        Region::across_all(
+            [self.name.region]
+                .iter()
+                .chain(self.vars.iter().map(|v| &v.region)),
+        )
+    }
+}
+
 #[derive(Debug, Clone, Copy, PartialEq)]
 pub enum Def<'a> {
     // TODO in canonicalization, validate the pattern; only certain patterns