Add circuitinfo CLI tool to count circuit constraints.

[porcuquine]

@nicola asked how many constraints are in one (presumably 32 GiB) Winning PoSt.

This PR adds a CLI tool, circuitinfo which counts constraints, public inputs, and partitions (as appropriate) for any circuit and sector size. Usage can be deduced from the example below. Available circuit types are --winning, --window, and --porep. Each desired type must be specified individually.

The example output below also contains the answer to the question at hand.

In the future, and as needed, this tool could be enhanced to provide other information about 'realistic' circuits.

Usage and all the relevant output for v28 circuits.

(base) ➜  rust-fil-proofs git:(feat/count-circuit-constraints) ✗ time cargo run --release --bin circuitinfo -- --winning --window --porep
    Finished release [optimized] target(s) in 0.22s
     Running `target/release/circuitinfo --winning --window --porep`
✔ Select the sizes for which constraints should be counted [use space key to select] ·  32 GiB,  64 GiB
Getting circuit info for sector size: 32 GiB
32 GiB Winning PoSt constraints: 370590, public inputs: 133, partitions: 1
32 GiB Window PoSt constraints (per partition): 125279217, public inputs (per partition): 25840, partitions: <depends on input size>
32 GiB PoRep constraints: 130278541, public inputs: 328, partitions: 10
Getting circuit info for sector size: 64 GiB
64 GiB Winning PoSt constraints: 391314, public inputs: 133, partitions: 1
64 GiB Window PoSt constraints (per partition): 129887900, public inputs (per partition): 25301, partitions: <depends on input size>
64 GiB PoRep constraints: 131185849, public inputs: 328, partitions: 10
cargo run --release --bin circuitinfo -- --winning --window --porep  828.92s user 0.78s system 99% cpu 13:52.19 total