Add GCRYMPI_FLAG_CONST and make use constants.
* src/gcrypt.h.in (GCRYMPI_FLAG_CONST): New. * src/mpi.h (mpi_is_const, mpi_const): New. (enum gcry_mpi_constants, MPI_NUMBER_OF_CONSTANTS): New. * mpi/mpiutil.c (_gcry_mpi_init): New. (constants): New. (_gcry_mpi_free): Do not release a constant flagged MPI. (gcry_mpi_copy): Clear the const and immutable flags. (gcry_mpi_set_flag, gcry_mpi_clear_flag, gcry_mpi_get_flag): Support GCRYMPI_FLAG_CONST. (_gcry_mpi_const): New. * src/global.c (global_init): Call _gcry_mpi_init. * mpi/ec.c (mpi_ec_ctx_s): Remove fields one, two, three, four, and eight. Change all users to call mpi_const() instead. * src/mpiutils.c (gcry_mpi_set_opaque): Check the immutable flag.
Allocating the trivial constants newly for every EC context is a waste
of memory and cpu cycles. We instead provide a simple mechanism to
internally support such constants. Using a new flag in THE API also
allows to mark an arbitrary MPI as constant. The drawback of the
constants is the their memory will never be deallocated. However,
that is what constants are about.