[feature]: decimal display support pt. 2

[jharveyb]

Addressing #925 , continuation of #930 .

• Only encode non-default (non-zero) precision values.
• Reject mints if the existing JSON metadata is too big to add the precision field to, if precision was specified.
• 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"
• display new precision field on some (?) RPC responses
  -- ListAssets, ListTransfers, and the address-related RPC calls

[dstadulis]

• Add guidance / explanation in documentation the rationale behind a decimal display / precision of four
  • Specify parity of the smallest unit
• Add recommended use of decimal display in documentation

[Roasbeef]

#1059 will fix the last two bullet points here

[Roasbeef]

Fixed by #1059