Transaction and Contract modules :: Add the signer as input

[Valazan]

Currently all the contract module functions (and some transaction-module functions) require signing, have the private key as an input. The idea is to replace the private key with a signer (an entity which is able to sign the transaction).

e.g.

    public load(
        address: string,
        abi: InterfaceAbi,
        callerPrivateKey?: string
    ): Contract {
        return new Contract(address, abi, this.thor, callerPrivateKey);
    }

Should become:

    public load(
        address: string,
        abi: InterfaceAbi,
        signer?: Signer
    ): Contract {
        return new Contract(address, abi, this.thor, signer);
    }

• [x] #820
• [ ] #821

[nwbrettski]

Dependency on #780