Batching or transaction-like capability for exported library methods

@anorth has suggested

> One consideration for improving the API is efficiency of state writes for bulk operations. The current API will force some unnecessary state.save() calls for intermediate states. E.g. intialising and then minting to a bunch of accounts up front, or making multiple transfers (from some non-standard token method). This API is pretty good for the basic cases and we want to keep them easy. We should think about how/whether to change this API to aid batching, or add an alternative style for less common but more complex calls. Maybe a closure that executes over state object, or a builder-style API that can abstract over it while batching.