1796: Support completion for macros r=matklad a=uHOOCCOOHu
This is based on #1795 , and fixes #1727
Also prettify hover text of macros.
Some screenshorts below:
Completion in item place.
<img width="416" alt="Screenshot_20190910_134056" src="https://user-images.githubusercontent.com/
14816024/
64587159-
fa72da00-d3d0-11e9-86bb-
c98f169ec08d.png">
After pressing `tab`.
<img width="313" alt="Screenshot_20190910_134111" src="https://user-images.githubusercontent.com/
14816024/
64587160-
fa72da00-d3d0-11e9-9464-
21e3f6957bd7.png">
Complete macros from `std`.
<img width="588" alt="Screenshot_20190910_134147" src="https://user-images.githubusercontent.com/
14816024/
64587161-
fb0b7080-d3d0-11e9-866e-
5161f0d1b546.png">
Hover text.
<img width="521" alt="Screenshot_20190910_134242" src="https://user-images.githubusercontent.com/
14816024/
64587162-
fb0b7080-d3d0-11e9-8f09-
ad17e3f6702a.png">
Co-authored-by: uHOOCCOOHu <hooccooh1896@gmail.com>