Move AbstractParty, AnonymousParty and Party into a new net.corda.core.identity package,
as they're not really cryptography tools, and in preparation for further code coming in
for identity.
Optimize imports on many files to clean up the resulting refactor.
* Make CompositeKey implement PublicKey
The initial implementation of composite keys as their own distinct class separate from PublicKey
means that the keys cannot be used on standard classes such as Certificate. This work is a beginning
to modifying CompositeKey to being a PublicKey implementation, although significant further work
is required to integrate this properly with the standard Java APIs, especially around verifying
signatures using the new key type.
* First stage of making CompositeKey implement PublicKey interface. Revert to using PublicKey everywhere we expect a key.
* Move algorithm and format into companion object (#432)
Move algorithm and format into companion object so that they can be referenced from other
classes (i.e. the upcoming signature class).
* Add simple invariants to construction of CompositeKey.
Builder emits CompositeKeys in simplified normalised form. Forbid keys with single child node, force ordering on children and forbid duplicates on the same level. It's not full semantical normalisation.
* Make constructor of CompositeKey private, move NodeWeight inside the class.
Add utility function for Kryo deserialization to read list with length constraints.
