add sliceKeys method on dictionary types

[bluesign]

Issue to be solved

context: https://discord.com/channels/613813861610684416/1275318858005348374

Currently Cadence has problem getting keys from big dictionaries ( for example NFT Collection's ownedNFTs )

To enumerate keys, we need to traverse all dictionary and load slabs from storage, where key and value is stored together.

I think it is possible to add a sliceKeys method, as atree knows about counts under index, it should be possible to skip some keys.

Suggested Solution

add a sliceKeys method to efficiently enumerate keys in a dictionary