Merkle Tree Rewrite

[bhgomes]

Things to Consider

• We might be able to get rid of BaseContribution since Partial might have this information
• ForkedTree contains enough information that we probably don't need the pointer logic
• We can get rid of Full in place of Partial
• Refactor Node/InnerNode API (we can probably get rid of it)
• Let's try to unify SentinelSource and InnerMap
• InnerPath should not have leaf information
• assert_valid_path should only take one pointer argument, probably index