This patch implements the R7RS 'syntax-error' macro, which supports
improved error reporting from within 'syntax-rules' macros.

For example:

  (define-syntax simple-let
    (syntax-rules ()
      ((_ (head ... ((x . y) val) . tail)
          body1 body2 ...)
       (syntax-error
        "expected an identifier but got"
        (x . y)))
      ((_ ((name val) ...) body1 body2 ...)
       ((lambda (name ...) body1 body2 ...)
        val ...))))
  
  (define (foo x)
    (simple-let ((y (* x x))
                 ((z1 z2) (values x x)))
      (+ y 1)))
  =>
  While compiling expression:
  ERROR: Syntax error:
  unknown location: simple-let: expected an identifier but got (z1 z2) in form (simple-let ((y (* x x)) ((z1 z2) (values x x))) (+ y 1))

This patch assumes that my earlier "custom ellipses" patch has already
been applied.  I'd like to push these to stable-2.0.

Comments and suggestions welcome.

      Mark