Extensible defpackage / define-package

This is a substantially redesigned version, with some incompatible changes.

define-package (previously define-conduit-package) is now the basic macro, with the old defpackage being a shim on top of it. define-package is now extensible: you can define your own mechanisms for handling clauses in its body. Both conduit packages and the underlying CL mechanism are defined like this and have no privileged status.