implemented `contains_prefix` for StorageDoubleMap and StorageNMap

[muraca]

In both StorageDoubleMap and StorageNMap is possible to delete or iterate on entries over a certain prefix, but there's no easy and optimized way of checking if at least one of those entries exists.

If you think I should add tests for StorageNMaps and point me where to add them, I'll be happy to do it.

[cla-bot-2021[bot]]

User @muraca, please sign the CLA here.

[shawntabrizi]

/tip small

[substrate-tip-bot[bot]]

@shawntabrizi A small tip was successfully submitted for muraca (12poSUQPtcF1HUPQGY3zZu2P8emuW9YnsPduA4XG3oCEfJVp on polkadot).

https://polkadot.js.org/apps/?rpc=wss%3A%2F%2Frpc.polkadot.io#/treasury/tips

[ggwpez]

Okay thanks! The CI failure is unrelated.

[muraca]

Any updates? Is it going to be merged?

[ggwpez]

bot merge

[paritytech-processbot[bot]]

Waiting for commit status.

[paritytech-processbot[bot]]

Merge cancelled due to error. Error: Statuses failed for f24a0ed7a4df67eee594a3bb6ef1c577d1507082

[Polkadot-Forum]

This pull request has been mentioned on Polkadot Forum. There might be relevant details there:

https://forum.polkadot.network/t/substrate-developer-newsletter-february-2023-notable-updates-for-frame-devs-new-deep-dives-released-calling-builders-to-apply-to-rfps-and-bounties/2137/1