Share more code between STM and Lin

[shym]

At the moment, STM and Lin share fairly little code. I see a few other opportunities that might be worth exploring:

• Spec in STM and CmdSpec in Lin share a lot of functionality, with slight mismatches (arbitrary vs separate Gen.t and Shrink.t)
• bridging that gap might open the way for generic commands in STM as in Lin_api, though I’m not sure that could be used without making next_state rather hard to write
• I think bringing something similar to STM’ ty to Lin would allow to build a safe equal_res.

(Some of those suggestions were open during a discussion with @n-osborne)