Add opticl-core and update opticl and retrospectiff

[slyrus]

I've created a new library opticl-core which contains the core types and pixel getter/setter functions and macros. This is now used by opticl and retrospectiff.

[quicklisp]

It seems to have an implicit dependency on alexandria. Can't build without making it explicit

[slyrus]

Ok, guess I need to got some sort of CI running... In the meantime, this is fixed. Thanks for pointing it out.

[slyrus]

Xach, does this work for you now? Can you remove the cantbuild label if so?