Enable clone()

[wbt]

As identified in Web3 Issue 3573, when using both TruffleContract and web3 functions it is important that they have separate Provider objects.

In the browser, the injected web3 has a web3.currentProvider which is a MetamaskInpageProvider, but passing this same object to a TruffleContract's setProvider function seems likely to produce issues like that one identified in web3.

To address this, the MetamaskInpageProvider might benefit from a clone() function producing another copy of the object, so that there can be independent queues serving overlapping sequences of requests from calls to the web3 object directly + calls to TruffleContract objects.