feat: propose() function

[ARR4N]

From propose() comment:

    /**
     * @notice "Announces" a propsed swap (in the form of a `Proposal` event), using the last block's hash as the salt
     * for the predicted swapper address.
     * @dev This is an optional step and swaps will function without prior proposal, but users SHOULD use this function
     * as it provides two security benefits: (i) a trusted means by which to safely determine the address they need to
     * approve to transfer their assets; and (ii) increasing the security level of the swapper address from 80 to 160
     * bits.
     * @dev A salt known to an adversary reduces the security of the swapper address to that of collision resistance
     * (~80 bits) whereas an unknown salt relies on second pre-image resistance (full address space = 160 bits). Using
     * the last block's hash would require computing a collision in the inter-block period (12s) so is sufficient.
     * @dev This function MAY be called on a different chain to the one on which the swap will occur, provided that the
     * deployer contract has the same address.
     */