Define a parser for various symbols in Scheme

`symbol :: Parser Char` is probably not needed (?)

I don't understand where the `Parser` type comes from, I cannot see it
in the doc of `Text.ParserCombinators.Parsec`. It may be that
`ParserCombinators` is a compatibility layer and that `Parser` is
defined somewhere else.

`oneOf` is provided by Parsec.