Restrict :version to systems

[Faré]

As part of https://bugs.launchpad.net/bugs/1465709 it was agreed that it would be a good idea to restrict :version so it be available only to systems, not to arbitrary components. Indeed, it doesn't make sense to do version checking inside a same source repository, where the source code is already in as coherent a state as it can ever be, as managed by git or another version control system. Version checking makes sense between loosely coupled systems. ASDF does not support addressing a component inside another system, and inasmuch as it does, should just use each component's system's version for any version tests.

restricting :version to systems, means (1) exporting a system-version gf, meant to replace component-version, (2) deprecating component-version, (3) finding all users of such versions in quicklisp, contact the authors so they stop using it, (4) removing the version slot from defclass component, move it to defclass system, and leave a stub for component-version, (5) removing component-version altogether. Probably a multi-year adventure (asdf feature propagation typically takes 2 years).