Support New Ledger Interface

[winder]

Problem

A small change in the ledger interface was introduced after adding the initial support for the new ledger. This is now preventing us from updating the submodule.

Solution

Implement new interface.

// LookupApplication loads an application resource that matches the request parameters from the ledger.
func (l *Ledger) LookupApplication(rnd basics.Round, addr basics.Address, aidx basics.AppIndex) (ledgercore.AppResource, error) {
	r, err := l.lookupResource(rnd, addr, basics.CreatableIndex(aidx), basics.AppCreatable)
	return ledgercore.AppResource{AppParams: r.AppParams, AppLocalState: r.AppLocalState}, err
}

// LookupAsset loads an asset resource that matches the request parameters from the ledger.
func (l *Ledger) LookupAsset(rnd basics.Round, addr basics.Address, aidx basics.AssetIndex) (ledgercore.AssetResource, error) {
	r, err := l.lookupResource(rnd, addr, basics.CreatableIndex(aidx), basics.AssetCreatable)
	return ledgercore.AssetResource{AssetParams: r.AssetParams, AssetHolding: r.AssetHolding}, err
}

Urgency

This will block a future release unless addressed.