KZG Key Generation, Save, Load

[Sun-Jc]

1. Faster KZG Setup using fixed base exponentiation comb (batch) and parallel powers of tau;

2. Save / load commitment keys (of KZG and Pedersen) to / from a ptau file, e.g. https://github.com/privacy-scaling-explorations/perpetualpowersoftau
   Closes issue Load existing KZG setup parameters #270

Benchmark

• Num of powers of taus: 2097152, i.e. $2^{21}$
• Computing powers of tau: 15.4ms
  • Baseline: 46ms
• Computation of fixed base exponentiation: 3.2s
  • Baseline: 24.3s
• File size: 128MB
• Save to file: 2.2s
• Load from file: 62ms
  • Baseline: 16.5s

Example to generate keys and save: cargo run -r --example kzg_keygen_tools

[Sun-Jc]

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