Merge #1815

1815: Support correct `$crate` expansion in macros r=uHOOCCOOHu a=uHOOCCOOHu

This PR makes normal use cases of `$crate` from macros work as expected.

It makes more macros from `std` work. Type inference works well with `panic`, `unimplemented`, `format`, and maybe more.
Sadly that `vec![1, 2, 3]` still not works, but it is not longer an issue about macro.

Screenshot:
![Screenshot_20190927_022136](https://user-images.githubusercontent.com/14816024/65714465-b4568f80-e0cd-11e9-8043-dd44c2ae8040.png)




Co-authored-by: uHOOCCOOHu <hooccooh1896@gmail.com>

crates/ra_syntax/src/ast/extensions.rs

@@ -21,6 +21,16 @@ impl ast::NameRef {
     pub fn text(&self) -> &SmolStr {
         text_of_first_token(self.syntax())
     }
+
+    pub fn as_tuple_field(&self) -> Option<usize> {
+        self.syntax().children_with_tokens().find_map(|c| {
+            if c.kind() == SyntaxKind::INT_NUMBER {
+                c.as_token().and_then(|tok| tok.text().as_str().parse().ok())
+            } else {
+                None
+            }
+        })
+    }
 }
 
 fn text_of_first_token(node: &SyntaxNode) -> &SmolStr {