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_raw(). The goal here is to remove any
     direct dependency of X509 on BIGNUM_C.

Changes
   * programs/x509/cert_write:
     - now it accepts the serial number in 2 different formats: decimal and
       hex. They cannot be used simultaneously
     - "serial" is used for the decimal format and it's limted in size to
       unsigned long long int
     - "serial_hex" is used for the hex format; max length here is
       MBEDTLS_X509_RFC5280_MAX_SERIAL_LEN*2