Add preliminary support for signed orders (off-chain auctions)

[reednaa]

• TODO: expose interface and validate signature

Goal:
Add support for an off-chain auction. That is an auction taking place on an order server where off-chain solvers bid. This PR does not define how the auction works. This can be defined in the future by setting the first byte of the fulfillment context.

Be warned:

1. The fulfillment context is overloaded. The user provides it as bytes1(TYPE) | bytes32(signer) | ... but the solver provides it as uint256(trueAmount) | bytes1(TYPE) | bytes32(signer) | ...
2. How trueAmount is derived is entirely up to the signer. It has to understand the type AND what the information after the signer means.
3. Based on CoinFillerWithFee.
4. Type 0x00 and 0x01 is not supported. It is expected that orders follow the structure above but it is not explicitly checked.

[reednaa]

The OutputDescription.amount should specify a minimum.

[reednaa]

Add an arbitrary 0x80 to the beginning of modified fulfillment context.

Add docs that means: 0x80 is 32 added bytes.