impl ECDSA-POD as introdction pod

[arnaucube]

benchmarks (i7-1270P 16cores thinkpad):

• (c1) ecdsa signature verification prove: 14s
• (c2) ecdsapod prove (verifies the proof from (c1)): 12.5s
• EcdsaPod total time: 40s
  • if first time: 60s (it has to compute for first time the verifier_datas, which would be parsed or computed beforehand)
• (for reference, proving the MainPod in the same machine takes 38s)

Depends on 0xPARC/pod2#256

[ed255]

This looks great! I've added a few suggestions and a request for an extra test, considering that this introduction pod will be used as a reference for other introduction pods.

[ed255]

LGTM!
This is a milestone! The first non-trivial pod verified by the Recursive MainPod!