ChangeLog.d/use_exp_mod_core.txt

Changes
   * mbedtls_mpi_exp_mod and code that uses it, notably RSA and DHM operations,
     have changed their speed/memory compromise as part of a proactive security
     improvement. The new default value of MBEDTLS_MPI_WINDOW_SIZE roughly
     preserves the current speed, at the expense of increasing memory
     consumption.