Rust: add missing keywords and minor improvements

Summary:
Changes:

• Add new missing keywords: https://doc.rust-lang.org/edition-guide/rust-2018/module-system/raw-identifiers.html#new-keywords
• Update the list of traits, with all core and std traits, obtained from the Rust documentation.
• Add keywords of constants.
• Remove duplicate keywords in different lists (such as self, Self and type).
• Highlight all symbols with the "Symbol" attribute.
• Highlight Raw identifier prefix (r#identifier).
• Highlight comments within definitions of functions and types.
• Correct indentation in some parts of the code and add modeline to avoid incorrect indentation (the file uses Tabs).

Sources:

• https://doc.rust-lang.org/core
• https://doc.rust-lang.org/std

Reviewers: Framework: Syntax Highlighting, dhaumann, cullmann

Reviewed By: Framework: Syntax Highlighting, dhaumann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D22848