Switch to ungrammar from ast_src

The primary advantage of ungrammar is that it (eventually) allows one
to describe concrete syntax tree structure -- with alternatives and
specific sequence of tokens & nodes.

That should be re-usable for:

* generate `make` calls
* Rust reference
* Hypothetical parser's evented API

We loose doc comments for the time being unfortunately. I don't think
we should add support for doc comments to ungrammar -- they'll make
grammar file hard to read. We might supply docs as out-of band info,
or maybe just via a reference, but we'll think about that once things
are no longer in flux

crates/ra_syntax/src/ast/expr_ext.rs

@@ -7,6 +7,8 @@ use crate::{
     SyntaxToken, T,
 };
 
+impl ast::AttrsOwner for ast::Expr {}
+
 impl ast::Expr {
     pub fn is_block_like(&self) -> bool {
         match self {