Merge Lexer and Syntax into Decoder; renames; Syntax class to type.

[galenhuntington]

• The Lexer and Syntax modules are merged into one, Decoder, since there doesn't seem need to separate them. As of now, all still have only pure functions.
• Other than that, nothing is yet moved except the egregiously misplaced Mode datatype, which is moved to State.hs.
• Renames:
  • The name Pretty for the machine-to-machine protocol to talk to mpg123 does not fit. I renamed to Cmd and have a function cmdToBS (not the best name maybe but okay).
  • I don't know why the single-letter Msg constructors don't match the single-letter codes from mpg123, and in fact confusingly remap them. I changed this so they match.
• I dropped the parsing of the Tag because nothing is done with it. Maybe in the future we'll want version sniffing or something but I have no plans for that.
• Cmd (formerly Pretty in Syntax) is demoted from a class to a datatype. I didn't see any advantage in the former, nor why the send protocol uses a class and the receive protocol a datatype. The code is simpler this way.
• The Info type is removed since it's an unhelpful wrapper around a ByteString.
• Minor UI adjustments while I was in there:
  • kbit/s is now kb/s.
  • If no track info is yet available, the entry in the upper-right is blank instead of saying (empty).
• Minor code tweaks.