Improvement: queryInlines -> foldInlines

[xarvh]

Right now the module has a queryInlines : (Inline i -> List a) -> Block b i -> List a.

An improvement on this, that would make the function more flexible and help with performance, could be to follow the fold pattern, ie:

foldInlines : (Inline i -> a -> a) -> a -> Block b i -> a

With this, queryInlines can be expressed in terms of this more generic function:

queryInlines f block = foldInlines (\inline acc -> f inline :: acc) [] block

This would allow the function to iterate on accumulators different than lists, avoiding intermediate allocations, and inserting more or less than one item per inline.
The example could be rewritten as:

    links : List String
    links =
        myMarkdownString
            |> Block.parse Nothing
            |> Block.foldInlines appendLink []

    getLinks : Inline i -> List String -> List String
    getLinks inline accum =
        case inline of
            Link url _ _ ->
                url :: accum
            _ ->
                accum