Add functions to inspect individual Indexable values

[adamgundry]

Given a single value of an Indexable type, it is possible to project out the values of any particular index for which IsIndexOf holds. However I can't figure out a way to do this very directly given the current API. The best I've been able to come up with is constructing and then consuming an IxSet, like this:

project :: forall ixs ix a . (IxSet.Indexable ixs a, IxSet.IsIndexOf ix ixs) => a -> [ix]
project a = map fst (IxSet.groupBy (IxSet.fromList @ixs [a]))

matches :: forall ixs ix a . (IxSet.Indexable ixs a, IxSet.IsIndexOf ix ixs) => [ix] -> a -> Bool
matches vs a = not $ IxSet.null (IxSet.fromList @ixs [a] IxSet.@+ vs)

It would be nice if ixset-typed exposed either these functions, or enough internals to make it possible to implement them. (At the moment there are quite a few things defined in Data.IxSet.Typed but not exported. Perhaps it is worth making a Data.IxSet.Typed.Internal module so the full definitions are accessible if needed?)