Add ephemeron tables

* libguile/ephemerons.h:
* libguile/ephemerons.c (scm_c_make_ephemeron):
(scm_c_ephemeron_key):
(scm_c_ephemeron_value):
(scm_c_ephemeron_mark_dead_x):
(scm_c_ephemeron_swap_x):
(scm_c_ephemeron_next): Add C ephemeron API.
(scm_make_ephemeron, scm_ephemeron_key, scm_ephemeron_value)
(scm_ephemeron_mark_dead_x): Dispatch to helpers above.
(scm_ephemeron_swap_x, scm_ephemeron_mark_dead_x): New Scheme-exposed
functions.
(scm_c_make_ephemeron_table):
(scm_c_ephemeron_table_length):
(scm_c_ephemeron_table_ref):
(scm_c_ephemeron_table_push_x):
(scm_c_ephemeron_table_try_push_x): New C API for tables of ephemerons.
(scm_ephemeron_table_length):
(scm_ephemeron_table_ref):
(scm_ephemeron_table_push_x):
(scm_ephemeron_table_try_push_x): New Scheme-exposed API.
(scm_c_ephemeron_hash_table_refq):
(scm_c_ephemeron_hash_table_setq_x):
(scm_c_ephemeron_hash_table_copy): New C API for use by internal weak
table users (dynamic states, etc).

* module/ice-9/ephemerons.scm: Add new Scheme API.

* libguile/evalext.c (scm_self_evaluating_p):
* libguile/goops.c (scm_class_of, %goops-early-init):
* libguile/print.c (iprin1):
* module/oop/goops.scm:
* libguile/scm.h (scm_tc7_ephemeron_table): Add new tc7 for ephemeron
tables.

* test-suite/tests/ephemerons.test ("ephemeron tables"): Add tests.

module/oop/goops.scm

@@ -70,7 +70,7 @@
             <vector> <bytevector> <uvec> <foreign> <hashtable>
             <fluid> <dynamic-state> <frame> <vm> <vm-continuation>
             <keyword> <syntax> <atomic-box> <thread> <bitvector>
-            <finalizer> <ephemeron>
+            <finalizer> <ephemeron> <ephemeron-table>
 
             ;; Numbers.
             <number> <complex> <real> <integer> <fraction>
@@ -1081,6 +1081,7 @@ slots as we go."
 (define-standard-class <bitvector> (<top>))
 (define-standard-class <finalizer> (<top>))
 (define-standard-class <ephemeron> (<top>))
+(define-standard-class <ephemeron-table> (<top>))
 (define-standard-class <thread> (<top>))
 (define-standard-class <number> (<top>))
 (define-standard-class <complex> (<number>))