SourcePos: Also track a character offset

[sjakobi]

The purpose of this parameter is to have character offset into the input stream.

Currently I use the following code to keep track of the offset:

handleNewline = do
  p0 <- P.getPosition
  _ <- P.char '\n'
  P.setPosition (P.incSourceColumn p0 1)

The downside of this addition is that while setSourceColumn and incSourceColumn could in principle also update the offset parameter, setSourceLine and setSourceLine cannot.