add support for variable transformers: settable identifier syntax

add support for variable transformers: settable identifier syntax

* module/ice-9/psyntax.scm (set!): Handle variable transformers; though,
  they cannot produce definitions.
  (make-variable-transformer): New procedure.
  (identifier-syntax): Allow the R6RS form that makes variable
  transformers.

* module/ice-9/psyntax-pp.scm: Regenerated.

* doc/ref/r6rs.texi (R6RS Incompatibilities): Remove letrec* item, and
  add set! restriction.