Address @strager's comments.

 * Operators (including syntax such as '->' and '\') are now colored as
   body text.

 * Commas and other delimiters are colored the same way as operators.

 * Character literals are now highlighted, and should interact correctly
   with primed identifiers.

 * Built-in types are now highlighted as types.

 * Escapes in strings and character literals are highlighted.

 * Floating-point numbers have the decimal point highlighted correctly.