Atomic Multi-Asset Transfers including ERC-20/721 tokens from a Smart Contract

As a smart contract developer, I want to take advantage of native multi-party multi-asset transfers where possible, without having to worry about whether one of those assets is an ERC-20/721 or not. I'd like to use a system contract that allows me to construct a transfer list including HBAR, HTS, and ERC tokens defined all together. The System contract will then do the dirty work -- call ERC tokens to perform transfers and then consolidate the remainder of the transfer list down to include just HBAR and HTS.

No files, tests, or entry points are named. Start by clarifying the system-contract interface and the expected handling of HBAR, HTS, and ERC-20/721 assets; done means the issue's combined transfer behavior and remainder handling are specified and covered by appropriate tests.

Written by the indexing model from the issue text.