During the unspecialized lambda set compaction procedure, we might end
up trying to merge too many disjoint variables during unspecialized
lambda unification. Avoid doing so, by checking if we're in the
compaction procedure.
Despite our best efforts, sometimes we still can't specialize lambda
sets on the fly, if a specialization lambda set's specialization type
isn't yet well-known! This commit adds an `AwaitingSpecializations`
data structure to keep track of the lambda sets blocked for
specialization behind a specialization's full resolution in the module.
After the specialization is resolved, its blocked lambda sets can be
eagerly compacted.
