Primary index tests.

This is a sequence of relatively simple property tests:

• Writing a primary index to disk and then reading it again is an identity operation (prop_write_load)
• We can create new primary indices by appending new entries to them (prop_open_appendOffsets_load)
• We can truncate primary indices to particular slot.
• Finding and reporting "filled slots" (not all slots in a chunk file, and hence in a primary index, need to contain a block) works as expected.
• Reconstructing a primary index from the same data results in the same primary index.

These property tests are QuickCheck based, which means they generate random indices, random slot numbers, etc., and come with a proper shrinker.