* add shortcuts for bytes32 and G1Element to test_proof_of_space.py to make it a bit easier to read
* introduce calculate_plot_difficulty(), for incrementing plot difficulty over time. Tweak the placeholder cut-over block heights to make it possible to test
* update verify_and_get_quality_string() to support more aspects of v2 plots. update the v2 proof verification stub to take plot difficulty. add plot filter test cases for v2 plots
* move some tests from weight-proof to proof-of-space