Improve the reader's token buffer mechanism.

- fewer special variables.
  Instead of *INCH-PTR* and *OUCH-PTR* there is just *READ-BUFFER*
  which is a struct, so among the simplifications, checking whether
  a recursive read is in progress is just one BOUNDP call.

- less consing through use of a pool of strings into which to read.

Reading single-floats (on 64-bit) and fixnums should never cons,
aside from making a new buffer one time.  Slurping in the contents
of enc-jpn-tbl.lisp is about 10% faster and conses 1/3rd as much.