Add disjoint syntax object type

* libguile/Makefile.am (libguile_@GUILE_EFFECTIVE_VERSION@_la_SOURCES):
  (DOT_X_FILES, DOT_DOC_FILES, noinst_HEADERS): Add syntax.c and
  syntax.h.
* libguile/evalext.c (scm_self_evaluating_p):
* libguile/goops.c (class_syntax, scm_class_of, scm_goops_early_init):
* libguile/init.c (scm_init_guile):
* libguile/print.c (iprin1):
* libguile/tags.h (scm_tc7_syntax):
* module/oop/goops.scm (<syntax>):
* module/system/base/types.scm (%tc7-syntax, cell->object):
* module/system/vm/disassembler.scm (code-annotation): Wire up the new
  data type.
* libguile/syntax.c:
* libguile/syntax.h: New files.
* module/ice-9/boot-9.scm: Move new definitions to (system syntax
  internal).
* module/system/syntax.scm (print-syntax): New helper.
* module/system/vm/assembler.scm (statically-allocatable?)
  (intern-constant, link-data): Arrange to be able to write syntax
  objects into images.
* module/language/cps/types.scm (&syntax): New type.  Remove
  &hash-table; it was never detected, an internal binding, and we need
  the bit to avoid going into bignum territory.

module/oop/goops.scm

@@ -62,7 +62,7 @@
             <boolean> <char> <list> <pair> <null> <string> <symbol>
             <vector> <bytevector> <uvec> <foreign> <hashtable>
             <fluid> <dynamic-state> <frame> <vm> <vm-continuation>
-            <keyword> <atomic-box>
+            <keyword> <syntax> <atomic-box>
 
             ;; Numbers.
             <number> <complex> <real> <integer> <fraction>
@@ -1009,6 +1009,7 @@ slots as we go."
 (define-standard-class <integer> (<real>))
 (define-standard-class <fraction> (<real>))
 (define-standard-class <keyword> (<top>))
+(define-standard-class <syntax> (<top>))
 (define-standard-class <atomic-box> (<top>))
 (define-standard-class <unknown> (<top>))
 (define-standard-class <procedure> (<applicable>)