Support Bulk Grant Contributions in a Single Transaction

[apbendi]

The current swapAndDonate method supports:

• Donating an arbitrary token
• To a single Grant
• In multiple Rounds

We want to update this to support:

• Donating an arbitrary token
• To multiple Grants
• In multiple denominations (e.g. 1 Grant receives 5 Dai and another 10 Dai)
• In multiple Rounds

To do this efficiently, it probably makes sense to first execute a single swap to donationToken, then transfer donationTokens to N Grant payee addresses. The assumption here is that 1 swap and N transfers is going to be less expensive than N swaps.

The caller should provide a proportion for each grant receiving a donation. The proportions should be applied to the total donationToken received after the swap.