Merge #1208

1208: [WIP] Goto for Macro's r=matklad a=Lapz

Adds goto definition for macros. Currently only works for macros in the current crate ~~otherwise it panics~~. Proper macro resolution needs to be added for it to resolve macros in other crates.

Todo
- [X] Allow goto from macro calls
- [X] Fix panics
- [x] Add tests



![Screen Recording 2019-04-25 at 18 00 24](https://user-images.githubusercontent.com/19998186/56754499-1dd01c00-6785-11e9-9e9a-1e36de70cfa3.gif)



Co-authored-by: Lenard Pratt <l3np27@gmail.com>

crates/ra_syntax/src/ast/generated.rs

@@ -1761,6 +1761,7 @@ impl ToOwned for MacroCall {
 
 impl ast::NameOwner for MacroCall {}
 impl ast::AttrsOwner for MacroCall {}
+impl ast::DocCommentsOwner for MacroCall {}
 impl MacroCall {
     pub fn token_tree(&self) -> Option<&TokenTree> {
         super::child_opt(self)