improve error message in Proof::new to include actual lengths

[ricecodekhmer]

Description:

I encountered an error in the last debugging session while initializing a new Proof using the Proof::new method. The error message simply was:

bail!("hash and path length mismatch");

This message did not include enough context to easily determine the problem. The error didn't state what lengths were being passed.

To improve this, I rewrote the error message to be more helpful. Now, if there is a mismatch, the error message will also indicate the actual lengths of the hash and the path. The new message is:

bail!("Proof::new: expected hash length = path.len() + 2, but got {} and {}", hash.len(), path.len());

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This change is