A first step in cleaning up RLP, which has lots of interesting issues -
the next step would be to clean up the exception handling as well
(Resultify?)
* remove `RlpNode` (unused)
* single-pass parsing for most functionality via RlpItem
* stricter conformance to spec
* remove float support
* warn about signed integers
* raise on invalid RLP earlier
* avoid several pointless allocations, in particular in `listLen`,
`listElem` etc
* include spec docs
Currently only setting `--styleCheck:hint` as there are some
dependency fixes required and the compiler seems to trip over the
findnode MessageKind, findnode Message field and the findNode
proc. Also over protocol.Protocol usage.