Rework promotions system

[chrean]

The current promotion system is not intuitive and presents a few issues, notably performance.

There are other problems, as noted in this PR: solidusio/solidus#4296 , which highlights a few solid points and aims at building a new system that we may swap in place of the old one, in time.
The idea here is to build an extension, then extrapolate the current system into its own extension, then finally swap the new extension in for the old one.