Use 'Display::fmt' instead of 'Error::description', fix warnings

[sharkdp]

Error::description has been deprecated in Rust 1.42: https://doc.rust-lang.org/stable/std/error/trait.Error.html#method.description

When compiling syntect with the latest compiler, error messages look like this:

[bat error]: /home/shark/Informatik/rust/bat/assets/syntaxes/01_Packages/PHP/PHP Source.sublime-syntax:
description() is deprecated; use Display

With this fix, we can see the reason for error messages again:

[bat error]: /home/shark/Informatik/rust/bat/assets/syntaxes/01_Packages/PHP/PHP Source.sublime-syntax:
Error while compiling regex '(?x)/\*\*(?:
  (?:\#@\+)?\s*(?m:$) (?# multi-line doc )
  |
  (?=\s+@.*\s\*/\s*(?m:$)) (?# inline doc )
)': Unknown group flag

I also fixed the other two remaining warnings concerning superfluous parentheses.

[sharkdp]

Note that using Display::fmt allows for a much greater flexibility in creating error messages (with Error::description, they had to be a &'static str, i.e. a fixed string literal).

[trishume]

Cool thanks for cleaning this up

[trishume]

I just released this and your other PR as v4.1.0

[sharkdp]

Thank you!