The old reader used an internal option to short-circuit special-value
reading when a special value acts as a terminating "character". Expose
that shortcut by allowing 'special as an argument to
`peek-{char,byte}-or-special`, and update the reader to use it.
Support an external implementation of `read-syntax` by exposing
functionality that is currently internal to `read-syntax`: a srcloc
argument to a "special"-producing port function and wrapping special
results to reliably distinguish them from characters.
