Update core.StateDiffs.StorageDiffs type

[rianhughes]

Update core.StateDiffs.StorageDiffs type to map[felt]map[key]value. This should help with merging stateDiffs

[omerfirmak]

In fact, we can get rid of pretty much all slices in StateDiff

IMO it should look something like;

type StateDiff struct {
	StorageDiffs      map[felt.Felt]map[felt.Felt]*felt.Felt
	Nonces            map[felt.Felt]*felt.Felt
	DeployedContracts map[felt.Felt]*felt.Felt // addr -> class hash
	DeclaredV0Classes map[felt.Felt]bool
	DeclaredV1Classes map[felt.Felt]*felt.Felt // class hash -> compiled class hash
	ReplacedClasses   map[felt.Felt]*felt.Felt  // addr -> class hash
}