Re-expose CryptokitBignum module

[mbacarella]

The CryptokitBignum module stopped being exposed after cryptokit was updated to build with dune. This breaks users that were depending on it, and requires pinning to cryptokit <v1.16.

Assuming that this module was not intentionally removed from the interface of the library, please kindly consider merging this PR.

[mbacarella]

I see that my editor helpfully removed trailing whitespaces from some lines and that my very OCaml aware diff tool didn't show that before I submitted.

Let me know if you'd like me to undo the trailing whitespace chopoffs. The only substantial change is the addition of module CryptokitBignum = CryptokitBignum.

[xavierleroy]

Before the switch to Dune, CryptokitBignum.cmi was installed along Cryptokit.cmi, making both visible.

You're proposing to make CryptokitBignum a sub-module of Cryptokit.

What about telling Dune to install CryptokitBignum.cmi like before?

[mbacarella]

Oh, right. My PR doesn't necessarily preserve backward compatibility as projects would now need to say open Cryptokit first or qualify it as Cryptokit.CryptokitBignum, whereas before they could just say CryptokitBignum.

Your suggestion would preserve the pre-dune behavior which would be nicer. Just... not sure what the right way of doing that with dune is.

[xavierleroy]

Here is a tentative fix: 08fe254 . Let me know if it does what you want.

[mbacarella]

Here is a tentative fix: 08fe254 . Let me know if it does what you want.

Works fine now. My PR is no longer necessary.

[mbacarella]

Thank you!