Create an error if we try to collapse E --> H

[BGluth]

As from the discussion in #202, if we ever have a branch where:

• It has exactly two children
• One child is a hash node
• One child is a leaf node
• The leaf node gets deleted

Then this is a bit of a problem as we can not collapse the remaining E --> H node if needed (because a branch with a single child collapses into an extension node). If the hash node was a leaf, then the leaf should "absorb" the extension node's key. However, we can not do this with a hashed leaf node.

It was discussed in Slack that if mpt_trie ever encounters an E --> H after a delete, then it should return an error. I don't think we should extend this to raise an error whenever we encounter one of these during traversal (eg. the pre-image that we were given contains a E --> H), as it's not a problem unless it's involved in a collapse. If we only return an error whenever we attempt to collapse an E --> H, then I think this should be sufficient. Does this sound right @Nashtare @frisitano?