Semantic highlighting spike

Very simple approach: For each identifier, set the hash of the range
where it's defined as its 'id' and use it in the VSCode extension to
generate unique colors.

Thus, the generated colors are per-file. They are also quite fragile,
and I'm not entirely sure why. Looks like we need to make sure the
same ranges aren't overwritten by a later request?

crates/ra_syntax/src/syntax_node.rs

@@ -523,6 +523,10 @@ impl<'a> SyntaxElement<'a> {
         }
     }
 
+    pub fn as_ast_node<T: AstNode>(&self) -> Option<&T> {
+        self.as_node().and_then(|x| <T as AstNode>::cast(x))
+    }
+
     pub fn as_token(&self) -> Option<SyntaxToken<'a>> {
         match self {
             SyntaxElement::Node(_) => None,