Improve the specialiser

This MR combines several bits of work:.

• Alexis King's work from !2913 (closed)
• A fix for #17966 (closed), by allowing partial specialisations
• A fix for #17810 (closed) (see the zapIdDemandInfo in specBind (the NonRec case).

I got sucked into some refactoring as well. The result is quite satisfying I think.

Fixes #17810 (closed), #17966 (closed), #17930 (closed)