Normally a module can see all of its abilities, and any module it
depends on (this is the tiny world, i.e. only the world it is aware of).
But when making specializations, a module may need to see all abilities
in all modules (big world). This patch supports big-world viewing of
abilities.
Now that we have unspecialized lambda sets, we need to properly copy
their specialization variables when a module imports another modules'
ability store. I think it will be easier to ensure the correctness if
we distinguish between a "pending" ability store (which may contain
imports whose variables have not been added to subs yet) and "resolved"
abilities stores, which are what we care about for solve and later on.
There is a cost to this since `PendingAbilitiesStore#resolve` destroys
and collects all of the ability member data, rather than in-place
mutating as we did before. However, the resulting `AbilitiesStore` is
now much smaller, so the memory footprint will be better, and also
every field besides `ability_members` will be moved rather than copied.
- Missing members are zero in the happy case
- Existing members will always be populated in the happy case, but I
don't think they improve the error message, so just get rid of it.
