several highlighting cleanups

* make stuff more type-safe by using `BindPat` instead of just `Pat`
* don't add `mut` into binding hash
* reset shadow counter when we enter a function

crates/ra_syntax/src/ptr.rs

@@ -60,6 +60,17 @@ impl<N: AstNode> AstPtr<N> {
     pub fn syntax_node_ptr(self) -> SyntaxNodePtr {
         self.raw
     }
+
+    // FIXME: extend AstNode to do this safely
+    pub fn cast_checking_kind<U: AstNode>(
+        self,
+        cond: impl FnOnce(SyntaxKind) -> bool,
+    ) -> Option<AstPtr<U>> {
+        if !cond(self.raw.kind()) {
+            return None;
+        }
+        Some(AstPtr { raw: self.raw, _ty: PhantomData })
+    }
 }
 
 impl<N: AstNode> From<AstPtr<N>> for SyntaxNodePtr {