inttostr: add a new function, inttostr, and tests

inttostr: add a new function, inttostr, and tests

The namesake function was not available.  The existence of the
template file, inttostr.c makes its addition nontrivial.
* lib/anytostr.c: Rename from inttostr.c.
(anytostr): Rename from inttostr.
* lib/inttostr.c: New file.
* modules/inttostr (Files): Add anytostr.c.
(Makefile.am): Set lib_SOURCES instead of ...
* m4/inttostr.m4: Remove uses of AC_LIBOBJ.
* lib/imaxtostr.c: Update use.  s/inttostr/anytostr/
* lib/offtostr.c: Likewise.
* lib/uinttostr.c: Likewise.
* lib/umaxtostr.c: Likewise.
* modules/inttostr-tests: New file.
* tests/test-inttostr.c: New file.  Test these functions.