Function varisat::clause::reduce::dedup_and_mark_by_tier

pub fn dedup_and_mark_by_tier(
    ctx: Mut<ClauseDbP, Mut<ClauseAllocP, Ref<Context>>>, 
    tier: Tier
)

Remove deleted and duplicate entries from the by_tier clause lists.

This has the side effect of setting the mark bit on all clauses of the tier.