Whoa, one more big commit. I didn't manage to untangle the
changes while working towards compatibility.

In some cases the iterator didn't properly return the correct key
because it didn't append fields to the reverse lookup.

* Reimplemented caching for trie
* Reimplemented resetting and persisting trie

* Fork tests (equal and larger chains)
* `chain.link` fields are now exported
* moved debug function from state to dump.go

Not sure if this is the right approach. Why? BlockChain shouldn't need
the "Ethereum" object. BlockChain shouldn't need to worry about
notifying listeners or message propagation.

* Block(s)
* Transaction(s)

rlp: new package for streaming RLP decoder

Tests are skipped due to reliance on deprecated mutan