Add checksums to Lance 2.1

[westonpace]

When using the mini-block encoding we can checksum each mini-block with a CRC32. Blocks should only be 4-8KiB and so CRC32 should provide pretty strong protection.

When using the full-zip encoding we will actually need to checksum each individual value. Since we don't use it until we hit 128 bytes then adding 4 bytes is probably not too detrimental.