Reimplement weak vectors in Scheme using ephemerons

* module/ice-9/weak-vector.scm: New implementation, same interface.

* doc/ref/api-memory.texi (Weak vectors): Default weak vector value was
documented as empty list when it was actually unspecified, but #f is
most useful, so we change documentation and code to match.

* libguile/Makefile.am (libguile_@GUILE_EFFECTIVE_VERSION@_la_SOURCES):
(DOT_X_FILES):
(DOT_DOC_FILES):
(noinst_HEADERS):
(modinclude_HEADERS):
* libguile.h:
* libguile/deprecated.c:
* libguile/deprecated.h:
* libguile/init.c:
* libguile/weak-vector.c:
* libguile/weak-vector.h: Remove C weak vector implementation, replaced
with deprecation stubs that call out to Scheme.

* libguile/weak-set.c:
* libguile/weak-table.c:
* libguile/weak-list.h: Remove unused internal header.

* libguile/eq.c:
* libguile/evalext.c:
* libguile/goops.c:
* libguile/hash.c:
* libguile/scm.h:
* module/system/base/types.scm:
* module/system/base/types/internal.scm:
* module/system/vm/assembler.scm: Remove wvect tc7.

libguile/weak-set.c

@@ -36,8 +36,6 @@
 #include "threads.h"
 #include "weak-set.h"
 
-#include "weak-list.h"
-
 
 /* Weak Sets