This also restores the old module structure of having separate
`peer_pool` and `rlpx` modules. This is made possible by the
new Nim package `package_visible_types` (please refer to its
README for an explanation).
Also introduces more error handling in the low-level RLPx routines.
All detected errors will result in immediate disconnection of the
corresponding peer, which may be detected in the protocols though
the `onPeerDisconnected` event handler.
In the next commit, I'm going to squash all the changes in the LES.
This commit just sets the directory tree to its final state, so the
diffs in the follow up commits are easier to review.
Please note that eth.nim was renamed to eth_protocol.nim, because
it's not advisable to have a module name that shares the name of
a protocol identifier (this creates hard to figure out ambiguity
errors)
