* tests/gpgscm/ffi-private.h: Handle character arguments.
* tests/gpgscm/ffi.c (do_string_index): New function.
(do_string_rindex): Likewise.
(do_string_contains): Likewise.
(ffi_init): Define new functions.
* tests/gpgscm/ffi.scm (ffi-define): New macro.
* tests/gpgscm/lib.scm (string-index): Use native function,
demonstrate behavior.
(string-rindex): Likewise.
(string-contains?): Likewise.
Demonstrate behavior of various other functions.
(read-all): Rework so that it can handle large files.