Some reader cleanups.

- Fewer special-variable references, slightly smaller/faster.

- Remove spurious double assignment of dispatch macros.

- Tokens returned from READ-EXTENDED-TOKEN[-ESCAPED] have not
  undergone case-conversion. The only place where it matters
  is for the #: reader, which calls CASIFY-READ-BUFFER.

- #\ and #* readers don't copy the read buffer.
Consistently pass a readtable to the character attribute testers.

This is marginally better for semantics and maybe performance.