API changes
   * The PSA and Mbed TLS error spaces are now unified. mbedtls_xxx()
     functions can now return PSA_ERROR_xxx values.
     There is no longer a distinction between "low-level" and "high-level"
     Mbed TLS error codes.
     This will not affect most applications since the error values are
     between -32767 and -1 as before.

Removals
   * Remove mbedtls_low_level_sterr() and mbedtls_high_level_strerr(),
     since these concepts no longer exists. There is just mbedtls_strerror().
