Add lazy share merging

[evan-forbes]

We want to solve issues such as #455, #454, and #77. This will require mechanisms to either estimate or accurately count the number of shares that a given set of transactions will actually take up. While we could create such mechanisms, they would be mirroring the share merging logic that we already have. It would likely be better if we could instead lazily add shares to a square. That way, instead of creating a new mechanism to count or estimate shares, we simply generate the shares as we go.

As a bonus, this will also set us up to solve #626, as we will already be generating the square during the PrepareProposal step, which will make generating the DataAvailabilityHeader and DataRoot trivial.