fix: allow to use blake3 to hash PV

[leruaa]

Motivation

verify() fails on proofs when sp1-zkvm blake3 feature is enabled.

Solution

Remove the blake3 feature everywhere except in sp1-zkvm, and when we want to verify PV, check with both Sha256 and Blake3 hashing.

[ratankaliani]

Add more documentation about the cross-collision infeasibility + use a helper function to increase code re-use.

In sp1-verifier we should also make it such that it checks both, rather than requiring a feature flag.

[ratankaliani]

Left some comments on functions that can be cleaned up. Can you also clean up the PR description?

[ratankaliani]

Left some suggestions for how to polish the documentation to be more readable.

Once addressed, will approve.

[ratankaliani]

LGTM