Move verify_share to return batch-verifiable statements

While the previous construction achieved n/2 average detection,
this will run in log2(n). Unfortunately, the need to keep entropy
around (or take in an RNG here) remains.

Cargo.lock

@@ -4631,8 +4631,8 @@ dependencies = [
  "hkdf",
  "minimal-ed448",
  "multiexp",
- "rand 0.8.5",
  "rand_chacha 0.3.1",
+ "rand_core 0.6.4",
  "schnorr-signatures",
  "serde_json",
  "subtle",