Bugfix
   * mbedtls_x509write_crt_set_serial() now explicitly rejects serial numbers
     whose binary representation is longer than 20 bytes. This was already
     forbidden by the standard (RFC5280 - section 4.1.2.2) and now it's being
     enforced also at code level.

New deprecations
   * mbedtls_x509write_crt_set_serial() is now being deprecated in favor of
     mbedtls_x509write_crt_set_serial_new(). The goal here is to remove any
     direct dependency of X509 from BIGNUM_C.