Change parsing to be more in line with R5RS

[tslawler]

Major changes:

• ++ is not a valid scheme identifier, so use string-append instead
• Use ; to denote comments rather than the Haskell --
• I left Haskell-style block comments in place, since
  1. they're useful, and
  2. they don't interfere with any other scheme syntax
• Use a few more niceties from Parsec to parse string literals, numbers

This still doesn't support things like hex/octal/binary numbers, but that shouldn't be too hard to add in the future.

[adamwespiser]

thanks!