Use "standard" unicode for conceals

[dag]

• For syntax, use the same symbols as -XUnicodeSyntax permits
  • http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#unicode-syntax
• For identifiers, use the same symbols as the base-unicode-symbols and containers-unicode-symbols packages provide
  • http://www.haskell.org/haskellwiki/Unicode-symbols

CC #45 #48