Use rustic changes that have not been applied to rust-mode.el

[brotzeit]

Recently rustic was changed to use rust-mode as dependency. Some new code in the former rustic.el was not moved to rust-mode.e(#407). We should merge those improvements:

• improved regexes in rustic-top-item-beg-re
• include keyword existential in rust-keywords
• performance improvements for rust-syntax-propertize functions (@phillord would be great if you could take a look)
• tests that are not present in rust-mode