load_balancing: more guidance needed for use of hash()

[liujed]

• Slide 78 of P4D2_2017_Fall/P4_tutorial_labs.pdf has information on the signature of hash() that should be copied into load_balancing.p4.
• Comments for ecmp_base and ecmp_count are needed in load_balancing p4. (Otherwise, I wouldn't know to use them for baseand max in the call to hash().)
• Hash algorithm is unspecified. (Need to say CRC16 somewhere.)
• Attempts based on example usage in PSA spec didn't work. My attempt:

Hash<bit<14>>(HashAlgorithm_t.CRC16) h;
meta.ecmp_select =
  h.get_hash({hdr.ipv4.srcAddr, hdr.ipv4.dstAddr,
    hdr.ipv4.protocol, hdr.tcp.srcPort, hdr.tcp.dstPort});

Resulting error:

syntax error, unexpected IDENTIFIER, expecting ;
        Hash<bit<14>>(HashAlgorithm_t.CRC16) h
                                             ^