Fix rules for chars

[MrEbbinghaus]

This PR addresses several issues regarding char detection.

Issues:

• Unicode chars with hexadecimal digits are not allowed.
• Unicode and octal chars with 1 or more digits are allowed.
  • Clojure accepts exactly four hex digits for Unicode
  • Octals must be between 0 and 377
• Octal chars with the digits 8 & 9 were allowed
• arbitrary special chars (e.g. \newline) are allowed
  • Clojure provides a fixed set for special chars

![\n\r\s] is not correct and results in: https://github.com/nextjournal/clojure-mode/issues/9

Actually, Clojure accepts \n and white spaces when in a form… Which is crazy…

Clojure 1.10.3
user=> \
Syntax error reading source at (REPL:2:1).
EOF while reading character
user=> [\ \a]
[\space \a]
user=> [\
 \a]
[\newline \a]
user=>

resolves https://github.com/nextjournal/clojure-mode/issues/22, resolves https://github.com/nextjournal/clojure-mode/issues/9

[mk]

@MrEbbinghaus thanks a lot for this!