The XLS-20 implementation contained two bugs that would require the
introduction of amendments. This complicates the adoption of XLS-20
by requiring a staggered amendment activation, first of the two fix
amendments, followed by the `NonFungibleTokensV1` amendment.
After consideration, the consensus among node operators is that the
process should be simplified by the introduction of a new amendment
that, if enabled, would behaves as if the `NonFungibleTokensV1` and
the two fix amendments (`fixNFTokenDirV1` and `fixNFTokenNegOffer`)
were activated at once.
This commit implements this proposal; it does not introduce any new
functionality or additional features, above and beyond that offered
by the existing amendments.
The existing code would, incorrectly, allow negative amounts in offers
for non-fungible tokens. Such offers would be handled very differently
depending on the context: a direct offer would fail with an error code
indicating an internal processing error, whereas brokered offers would
improperly succeed.
This commit introduces the `fixNFTokenNegOffer` amendment that detects
such offers during creation and returns an appropriate error code.
The commit also extends the existing code to allow for buy offers that
contain a `Destination` field, so that a specific broker can be set in
the offer.
o Fixes an off-by-one when determining which NFTokenPage an
NFToken belongs on.
o Improves handling of packed sets of 32 NFTs with
identical low 96-bits.
o Fixes marker handling by the account_nfts RPC command.
o Tightens constraints of NFTokenPage invariant checks.
Adds unit tests to exercise the fixed cases as well as tests
for previously untested functionality.
