Syntax highlighting improvements, message features and fixes

• Syntax:
  • Support irrefutable patterns in closure and function params #349
  • Fix raw pointers #335
  • Index const and static symbols (for Goto-Symbol) #335
  • Support raw identifiers (new rust 1.30) #338
  • Fix tuple access so it doesn't show up as a float. #348
  • Support vis macro designator (new rust 1.30) #347
• On-save check:
  • Show result in window status when done. #345
  • Fix on-save checking being triggered multiple times with "Save All" and the build command when the buffer is unsaved. #344
• Diagnostic messages: #344
  • Support inline rustdoc messages with cargo doc (requires rust 1.30) #334
  • Add rust_message_status_bar option to display the message under the cursor in the window status bar.
  • Add rust_message_popup command to manually show a popup under the cursor.
  • Add underline styles to rust_region_style.
  • Fix various issues with multiple views into the same file.
  • Fix Next/Prev when a message is hidden (such as a completed suggestion).
  • Fix hitting Esc in the middle of the build to hide/dismiss future messages.
• Snippets: honor indentation settings #342