Add extensions to several asset operations

[abitmore]

For BSIP 48, 75 and 77

Features:

• price feed producers can publish ICR (supplement to BSIP77)
• asset owners can update MCR and MSSR directly (BSIP75) (also ICR due to BSIP77)
• asset precision can be updated when asset has zero supply (BSIP48)
  • precision of prediction markets can't be updated
  • if any other asset is backed by this asset, this asset's precision can't be updated
• when updating asset, can skip updating CER (core_exchange_rate), this is useful for updating via proposals which take time to approve (may be put into BSIP48)
• new flags for assets (BSIP48)
  • whether max supply can be updated
  • whether new supply can be created (issued by asset owner for UIAs, or created by borrowers for MPAs/PMs)
• new asset issuer permissions (to be forfeited by asset owners) (BSIP48/75/77)
  • to update the new "whether max supply can be updated" flag
  • to update the new "whether new supply can be created" flag
  • for MPAs/PMs, to update MCR
  • for MPAs/PMs, to update ICR
  • for MPAs/PMs, to update MSSR
• fix a bug about flags
  • before the hard fork, unknown bits can be set (to 1) in flags when creating an asset, and can not be unset (to 0) when updating the asset (and the non-buggy part: can not set the unknown bits when updating);
  • after the hard fork, when creating an asset, neither the invalid bits nor the unknown bits can be set in flags;
  • after the hard fork, when updating an asset, neither the invalid bits nor the unknown bits can be set in flags; if a bit was already set, when updating the asset, it must be unset.
• fix a bug about max_supply
  • before the hard fork, can update an asset's max supply to a number smaller than current supply
  • after the hard fork, can't update an asset's max supply to a number smaller than current supply
• fix a bug about global_settle permission of prediction markets
  • before the hard fork, the global_settle permission of prediction markets can be disabled via asset_update_operation, if done so, it can't be enabled again if supply is not zero;
  • after the hard fork, when updating a prediction market, the global_settle permission must be enabled in new_options, and it's allowed to be enabled if was disabled before the hard fork.
• Independent asset creation fee for assets with a prefix

TODO:

• test case for new precision
• test case for ICR/MCR/MSSR

[abitmore]

Ready for review.