Formatting a byte slice or array with %v prints one decimal number per
byte, such as "[167 190 112 ...]". Digests, seeds, VRF proofs and
voting keys in error and panic messages now print as hex instead.

Each value is passed as a plain []byte, either sliced or converted, so
fmt takes its fast path rather than reflecting over a named type.
StateProof.String also drops its strings.Builder helper.

Apple M2 Pro, fmt.Sprintf of one value:

  StateProof.String        2926ns, 71 allocs -> 230ns, 4 allocs
  Seed [32]byte            1101ns -> 94ns (196ns with %x on Seed)
  VrfProof [80]byte        2607ns -> 162ns (381ns with %x on VrfProof)
  GenericDigest (32 bytes) 1392ns, 34 allocs -> 93ns, 2 allocs

Signature, PublicKey, the ed25519 signature and public key types, and
merklesignature.Commitment now print as hex. These appear in every
SignedTxn, multisig, vote and keyreg transaction, so error messages
that format those structs with %+v are shorter and faster.

PublicKey's String method is promoted to SignatureSecrets through its
embedded SignatureVerifier, so %v of SignatureSecrets now prints only
the public key instead of the private key bytes.

Rather than each call site remembering to use %x with a slice, the
types now format themselves. GenericDigest, Sha512Digest, VrfProof,
VrfPubkey, OneTimeSignatureVerifier and committee.Seed print as hex
with %v or %s, so the call sites changed in the previous commit go
back to plain %v.

The remaining sites format unnamed byte arrays and slices, so they use
%x directly. Slicing an array first would skip fmt's reflection path,
but these are error paths and the plain form reads better. The lease
error prints base64 instead, matching goal's --lease flag and the REST
API.