Define top-level bindings for aux syntax: else, =>, _, ...

* module/ice-9/boot-9.scm (else, =>, ..., _): New definitions.  These
  are specified by the r6rs and the r7rs.
* module/ice-9/sandbox.scm (core-bindings): Include the aux syntax
  definitions.
* module/rnrs/base.scm:
* module/rnrs.scm: Re-export aux syntax.

module/ice-9/boot-9.scm

@@ -416,6 +416,22 @@ If returning early, return the return value of F."
 (define-syntax-rule (unless test stmt stmt* ...)
   (if (not test) (begin stmt stmt* ...)))
 
+(define-syntax else
+  (lambda (x)
+    (syntax-violation 'else "bad use of 'else' syntactic keyword" x x)))
+
+(define-syntax =>
+  (lambda (x)
+    (syntax-violation '=> "bad use of '=>' syntactic keyword" x x)))
+
+(define-syntax ...
+  (lambda (x)
+    (syntax-violation '... "bad use of '...' syntactic keyword" x x)))
+
+(define-syntax _
+  (lambda (x)
+    (syntax-violation '_ "bad use of '_' syntactic keyword" x x)))
+
 (define-syntax cond
   (lambda (whole-expr)
     (define (fold f seed xs)

module/ice-9/sandbox.scm

@@ -314,6 +314,7 @@ allocation limit is exceeded, an exception will be thrown to the
 ;;
 (define core-bindings
   '(((guile)
+     else => _ ...
      and
      begin
      apply

module/rnrs.scm

@@ -1,6 +1,6 @@
 ;;; rnrs.scm --- The R6RS composite library
 
-;;      Copyright (C) 2010, 2011 Free Software Foundation, Inc.
+;;      Copyright (C) 2010, 2011, 2019 Free Software Foundation, Inc.
 ;;
 ;; This library is free software; you can redistribute it and/or
 ;; modify it under the terms of the GNU Lesser General Public
@@ -73,7 +73,7 @@
 	  vector-map vector-for-each error assertion-violation assert
 	  call-with-current-continuation call/cc call-with-values dynamic-wind
 	  values apply quasiquote unquote unquote-splicing let-syntax 
-	  letrec-syntax syntax-rules identifier-syntax
+	  letrec-syntax syntax-rules identifier-syntax else => _ ...
 
 	  ;; (rnrs bytevectors)
 	  

module/rnrs/base.scm

@@ -1,6 +1,6 @@
 ;;; base.scm --- The R6RS base library
 
-;;      Copyright (C) 2010, 2011 Free Software Foundation, Inc.
+;;      Copyright (C) 2010, 2011, 2019 Free Software Foundation, Inc.
 ;;
 ;; This library is free software; you can redistribute it and/or
 ;; modify it under the terms of the GNU Lesser General Public
@@ -23,7 +23,7 @@
 	  define define-syntax syntax-rules lambda let let* let-values 
 	  let*-values letrec letrec* begin 
 
-	  quote lambda if set! cond case 
+	  quote lambda if set! cond case else => _ ...
 	 
 	  or and not