mpi: Add an API for EC point operations.
* mpi/ec.c (gcry_mpi_point_new, gcry_mpi_point_release): New. (gcry_mpi_point_get, gcry_mpi_point_snatch_get): New. (gcry_mpi_point_set, gcry_mpi_point_snatch_set): New. * src/visibility.h, src/visibility.c: Add corresponding macros and wrappers. * src/gcrypt.h.in (struct gcry_mpi_point, gcry_mpi_point_t): New. (gcry_mpi_point_new, gcry_mpi_point_release, gcry_mpi_point_get) (gcry_mpi_point_snatch_get, gcry_mpi_point_set) (gcry_mpi_point_snatch_set): New prototypes. (mpi_point_new, mpi_point_release, mpi_point_get, mpi_point_snatch_get) (mpi_point_set, mpi_point_snatch_set): New macros. * src/libgcrypt.vers (gcry_mpi_point_new, gcry_mpi_point_release) (gcry_mpi_point_get, gcry_mpi_point_snatch_get, gcry_mpi_point_set) (gcry_mpi_point_snatch_set): New symbols. * src/libgcrypt.def: Ditto. * tests/t-mpi-point.c: New. * tests/Makefile.am (TESTS): Add t-mpi-point