Merge #4700

4700: Add top level keywords completion r=matklad a=mcrakhman

This fixes the following issue: #4566. 

Also added simple logic which filters the keywords which can be used with unsafe on the top level.   

Co-authored-by: Mikhail Rakhmanov <rakhmanov.m@gmail.com>
Co-authored-by: Aleksey Kladov <aleksey.kladov@gmail.com>

Author: 3 people

crates/ra_ide/src/completion.rs

@@ -15,6 +15,7 @@ mod complete_unqualified_path;
 mod complete_postfix;
 mod complete_macro_in_item_position;
 mod complete_trait_impl;
+mod patterns;
 #[cfg(test)]
 mod test_utils;