Add header field for number of shares used

[adlerjohn]

#80 explored the performance of different square sizes, and determined that the number of shares for each row and column of the available data square should always be a power of 2. This could result in a lot of wasted computation and sampling if only slightly more than a quarter of the shares are used for real data (indeed, this is exactly what prompted #68).

To avoid waste, we can do two things:

1. Add the number of shares used for "real" data in the block header.
2. Since any shares beyond the number of real data shares must be tail padding, rows containing only tail padding should be omitted from the row NMT commitments, and don't have to be sampled. The exact number of rows to omit can be deterministically computed using the current available data square size and the number of real data shares.