Adds a couple utility function to p2p server to check whether
we're already connected to a peer before trying again. Other
very minor fixes and logging improvements.
* core: cleanup slicing impls for Hash
* core: clean up Readable trait, implement Readable/Writeable for various integers
* core: change Hash debug output to hex
* core: correct warnings in all modules
* Replace AsFixedBytes with Sized + AsRef<[u8]>
* Add AsRef<u8> to impl_array_newtype!
* Include AsFixedBytes as marker trait
* Related fixes
* Remove Deref