Add collapse strategy to PartialTrie variants

[Nashtare]

Add a collapsing strategy for the edge case of a branch node with two children being collapsed into an extension node after the deletion of one the children.

cf #202 and #455 for reference

@0xaatif I don't consider the PR ready yet, especially some more doc/clarification would be good but I'm opening it to get some feedback regarding the approach taken and if it matches how you were envisioning it following our chat.

[BGluth]

Hey just for some more info, how are we handling this in the decoder? Isn't this something that must be done in the full node? Like if the decoder realizes that an extension will collapse into a hash node, doesn't it not know anything about what node was hashed, since the only case where this is safe is if the hash node was a branch node IIRC?

[Nashtare]

Jerry confirmed that this is always handled by Jerigon, hence the rationale behind ignoring these edge-cases from Jerigon payloads and only trigger errors from native client ones.
I don't see how unmodified nodes can alleviate this though, as relying only on regular RPCs like eth_getStorageAt or eth_getProof won't suffice

[BGluth]

Jerry confirmed that this is always handled by Jerigon, hence the rationale behind ignoring these edge-cases from Jerigon payloads and only trigger errors from native client ones.

@Nashtare Ah ok. So, if I'm understanding this right, even if Jerigon is handling this, under normal circumstances, we should never trigger the logic to collapse a extension node into a hash node right? Isn't it maybe better to still treat that as an invalid state even if we should still never be able to reach it (eg. like the unreachable! macro in rust)?

[Nashtare]

Not quite, with jerigon payloads, Jerry guaranteed that having this orphaned hash node converted to an extension node will never cause inconsistent trie root, unlike what we were having on frisitano's issue. I have Jerigon payload examples that were triggering these false positives, but were sane to handle for hte prover.

[BGluth]

Ah gotcha, makes sense!