Use a function over a closure to handle poor inference

[jakobnissen]

The previous code pattern using takewhile relied on Julia's type inference to produce correct results. In particular, it relied on takewhile(f, x) |> collect to correctly infer to Vector{<:Tuple{Char, Any}}. However, type inference in Julia is an optimization, and should not be relied on to produce the correct answer.
Instead, use a normal function to replace the functional style of programming.

Edit: I haven't been able to run tests locally, so this might not work as-is. See #63

[tecosaur]

At a glance, this looks good, but it seems to be made against an old version of the repo?

[jakobnissen]

Edit: Should be solved now.
This fails because the doctests run in a module which doesn't have access to read_while!. I don't understand why the doctests run in a different module from where the function is defined.
You can delete the docstring examples, but I'd think it was better if the problem was solved.