Rework substrate mock runtime

[xermicus]

Complete rewrite of the substrate mock runtime:

• Updates to a wasmi version with the new API (that's a breaking change requiring some substantial changes anyways)
• Reworked the API of MockSubstrate to hide all logic behind functions. This reduces the API surface to a set of functions on a struct, which will make future changes easier.
• Re-implement the host functions so that they resemble up-to-date pallet-contracts more closely
• Introducing separation between Wasm modules (= code), contracts (= code + storage) and accounts (= address +balance + optional contract). This lines the mock implementation up a bit closer to how real nodes work, making implementing additional features in the future easier.

I uncovered a bug in the process. The following contract (adapted from the calls::try_catch_constructor test) traps the execution with MemoryOutOfBounds:

contract c {
    constructor() payable {}

    function test() public returns (int32) {
        int32 x = 0;
        try new other(true) {
            x = 1;
        } catch (bytes memory _c) {
            x = 2;
        }
        return x;
    }
}

contract other {
    constructor(bool rev) public {
        if (rev) {
            revert("foo");
        }
    }

    function _ext() public {}
}

This code also fails on a real substrate contracts node (though this returns just ContractTrapped, obscuring the underlying reason).. Hence I concluded that the out of bounds memory access most likely stems from a bug in the compiler and disabled the failing test for now.

[seanyoung]

Great, this code is much, much better!

[seanyoung]

seal_tranfer no longer cheecks that the 2nd and 4th argument are correct; we should ensure that our code sets the correct length, else it will not work on a real node.

[xermicus]

Good point, I added those checks

[seanyoung]

The old code ensured that _flags was set to zero

[xermicus]

Thanks, added the check again