Deprecate srfi-14 in default environment

* libguile/srfi-14.c:
* module/srfi/srfi-14.scm: Arrange to define bindings here instead of in
the default environment.
* libguile/init.c:
* libguile/srfi-14.h: Fix so that srfi-14 can register an extension to
be loaded later.
* libguile/unidata_to_charset.awk: Rename private cs_letter_plus_digit
to cs_letter_and_digit.
* module/ice-9/deprecated.scm: Add deprecation trampolines for srfi-14.
* module/ice-9/sandbox.scm:
* module/language/elisp/lexer.scm:
* module/language/scheme/decompile-tree-il.scm:
* module/rnrs/unicode.scm:
* module/sxml/simple.scm:
* module/system/base/syntax.scm:
* module/system/repl/server.scm:
* module/texinfo/serialize.scm:
* module/texinfo/string-utils.scm:
* module/web/http.scm:
* module/web/uri.scm:
* test-suite/tests/strings.test:
* test-suite/tests/tree-il.test: Adapt for srfi-14 being in a module.

libguile/init.c

@@ -444,7 +444,7 @@ scm_i_init_guile (struct gc_stack_addr base)
   scm_init_read ();
   scm_init_strorder ();
   scm_init_srfi_13 ();
-  scm_init_srfi_14 ();  /* Requires smob_prehistory */
+  scm_boot_srfi_14 ();  /* Requires smob_prehistory */
   scm_init_exceptions ();
   scm_init_throw ();    /* Requires smob_prehistory */
   scm_init_version ();