changes to nfts pallet for xcm integration (#14395)

* Use Incrementable from frame_support::traits

* Chore

* make incremental fallible and new nfts function for custom collection ids

* fmt

* fix benchmark tests nfts

* add test

* fmt

* add safety comment to CollectionId

* fmt

* add comments to Incrementable

* line wrapping

* rewrap comments

* address feedback

* fmt

* change unwrap for expect

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Co-authored-by: Jegor Sidorenko <jegor@parity.io>
Co-authored-by: parity-processbot <>