Lift requirement that releaseParams() be called before tailCall() or allowCancellation() -- this is no longer necessary given the protocol simplification that allowed cap descriptors to be interpreted immediately upon receipt.
Add pkg-config support.
Define additional type aliases AnswerId and ImportId.  In the RPC protocol, always use the type corresponding to the sender's point of view.  In the implementation, always use the type corresponding to the local point of view.
Eliminate the concept of imbuing messages in favor of the simpler concept of setting a cap table directly on MessageReader / getting one from MessageBuilder.  This eliminates capability-context entirely.  This was made possible by the earlier change which moved capabilities to a separate table rather than storing CapDescriptors inline, but I didn't realize it at the time.