Make TxReceipt polymorphic

[ndzik]

Currently this is required:

		const tx = await session.mint(nftAddress, nftId);
		const minttx = tx.tx as Mint;

I think it would be nice to have a polymorphic TxReceipt<T extends Transaction> helper, which types the internal transaction:

		const minttx: TxReceipt<Mint> = await session.mint(nftAddress, nftId);

This can then be set in the return values of our interfaces and should be straight forward to implement.

[sebastianst]

I like the idea of creating a templated type-safe TxReceipt type. But I think that it is still better to have two separate fields tx for the transaction and deltas for the modified deltas.