Utility methods for slab keys

[janezpodhostnik]

Issue To Be Solved

It would be convenient (and safer) if cadence exposed methods for recognizing if a storage key is a slab key, so that code importing cadence doesn't have to know about the slab key structure.

This was relevant here: onflow/flow-go#3644

Suggested Solution

add function:

func IsSlabIndex(key string) bool {
	return len(key) == 9 && key[0] == '$'
}

[turbolent]

This mapping from a slab index to a ledger key is implemented in atree:

atree/storage.go

Lines 193 to 195 in e6b288c

	func SlabIndexToLedgerKey(ind StorageIndex) []byte {
	return []byte(LedgerBaseStorageSlabPrefix + string(ind[:]))
	}

atree already provides a function LedgerKeyIsSlabKey:

atree/storage.go

Lines 197 to 199 in e6b288c

	func LedgerKeyIsSlabKey(key string) bool {
	return strings.HasPrefix(key, LedgerBaseStorageSlabPrefix)
	}

.

However, it only checks for the prefix, not the length. It could maybe be improved.

Should I move the issue?