Since [inG] ranges over [cmraT]s, using an [ucmraT]s results in
[ucmra_cmraR] coercions that slow down type checking.

This commit improves the compilation time of thread_local.v by 40%.

This makes stuff more uniform and also removes the need for the [inGFs]
type class. Instead, there is now a type class [subG Σ1 Σ2] which expresses
that a list of functors [Σ1] is contained in [Σ2].

Use it to prove that tests/barrier_client and tests/heap_lang are adequate.