Fix error messages involving definition forms

Fix error messages involving definition forms

* module/ice-9/psyntax.scm (syntax-type): Return an additional value
  that contains the entire form in _all_ cases, including for definition
  forms.  Previously, the entire form was not returned for definition
  forms.

  (expand-expr): Add an additional argument that contains the entire
  form in _all_ cases, including for definition forms.  Use it to
  include the entire form in error messages, notably for definitions in
  expression context.  Include the source location information, which
  was previously missing from these errors when the rhs expression was
  an atom.  Improve the "definition in expression context" error message
  to be more comprehensible for Scheme beginners.

  (expand-top-sequence, expand, expand-body): Adjust as needed to handle
  the additional return value from 'syntax-type' and the additional
  argument to 'expand-expr'.

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

* NEWS: Update.