add alt_flat function

[EmileTrotignon]

Add an alt_flat function, which is just like alt but for regexps of the same type.

This PR together with #27 and #28 allow to write the following code :

let str_nbsp txt = Tyre.(attach (" " ^ txt) (str " " *> str txt))

let insert_nbsp_in_string txt =
  Result.get_ok
    Tyre.(replace (compile (str_nbsp ":" <||> str_nbsp ";" <||> str_nbsp "!")) txt)

Neat !

[Drup]

This is the signature of the traditional alt for an applicative functor .... The problem is that the behaviour, when combined with conv, is not very consistent. In one direction, it "tries both branches" and use the one that works. In the other direction, it just takes the left in an arbitrary fashion. I think it would definitely make for a footgun.

Do you have a use case that can't be solved by using raw Re (and Tyre.regexp) ?

[EmileTrotignon]

I did not realize it meant eval could return a value that is inconsistent with the input.

I do not need raw Refor this, conv and regular alt are enough to reimplement it.

I still want this in Tyre, but I believe the correct way to go about it is to return an exception in conv's from argument, and update the documentation accordingly.

In a lot of cases, regexps are never going to be evaluated, so I think it is fine if its properly documented. I can also add an explanation that explains that you need one alt_flat per type, handwritten with conv if you need alt_flat and evaluation.