[x] Only encode non-default (non-zero) precision values.
[x] Reject mints if the existing JSON metadata is too big to add the precision field to, if precision was specified.
[x] On reissuance, fetch the precision from the asset metadata (if possible) of the asset group anchor, and assert equality with the reissuance request.
-- "Precision should be fixed for all issuances in an asset group"
[x] display new precision field on some (?) RPC responses
-- ListAssets, ListTransfers, and the address-related RPC calls
Addressing #925 , continuation of #930 .