A utility to print the powers of tau from a challenge file

[weijiekoh]

This PR introduces a print_powers program that reads a challenge file and outputs the powers of tau stored in it. It is useful for anyone who wants to use these values for purposes such as polynomial commitment libraries.

Usage: 
<challenge_file> <circuit_power> <batch_size> <category_to_extract> <num_to_extract>

<category_to_extract> should be one of:

• taug1
• taug2
• alphag1
• betag1
• betag2

For example, this command prints the first 2 TauG2 elements stored in challenge. Note that challenge was generated using a circuit power of 28 and batch size of 256:

phase2-bn254/powersoftau$ ./target/release/print_powers challenge 28 256 taug2 2
G2(x=Fq2(Fq(0x1800deef121f1e76426a00665e5c4479674322d4f75edadd46debd5cd992f6ed) + Fq(0x198e9393920d483a7260bfb731fb5d25f1aa493335a9e71297e485b7aef312c2) * u), y=Fq2(Fq(0x12c85ea5db8c6deb4aab71808dcb408fe3d1e7690c43d37b4ce6cc0166fa7daa) + Fq(0x090689d0585ff075ec9e99ad690c3395bc4b313370b38ef355acdadcd122975b) * u))
G2(x=Fq2(Fq(0x04c5e74c85a87f008a2feb4b5c8a1e7f9ba9d8eb40eb02e70139c89fb1c505a9) + Fq(0x21a808dad5c50720fb7294745cf4c87812ce0ea76baa7df4e922615d1388f25a) * u), y=Fq2(Fq(0x2d58022915fc6bc90e036e858fbc98055084ac7aff98ccceb0e3fde64bc1a084) + Fq(0x204b66d8e1fadc307c35187a6b813be0b46ba1cd720cd1c4ee5f68d13036b4ba) * u))

Note how in the above example, the first TauG2 element is the generator of the G_2 group as specified in EIP-197.