Add types to some comprehensions

Currently the correctness of this depends on the exact typejoin logic
for comprehensions. That's probably reasonably safe, but it can break
when developing Base, and since this is a stdlib, let's be a good
citizen and annotate the types.

Author: Keno

src/regioniterator.jl

@@ -46,7 +46,7 @@ function eachregion(s::AnnotatedString, region::UnitRange{Int}=firstindex(s):las
                                             for region in first.(s.annotations)) |>
                                                 unique |> sort)
     isempty(changepoints) &&
-        return RegionIterator(s.string, [region], [map(last, annotations(s, first(region)))])
+        return RegionIterator(s.string, UnitRange{Int}[region], Vector{Pair{Symbol, Any}}[map(last, annotations(s, first(region)))])
     function registerchange!(start, stop)
         push!(regions, start:stop)
         push!(annots, map(last, annotations(s, start)))