[dkml] Virtual library for mirage-crypto-pk

[jonahbeckford]

TLDR: Having a virtual library so that GMP is not a hard-dependency of mirage-crypto-pk.

This was forked off from #137.

The specific request:

• Create a virtual library (https://dune.readthedocs.io/en/stable/variants.html). Let's call it mirage-crypto-bigint for now.
• Inside it has a mirage_crypto_bigint.mli specifying exactly what types and functions mirage-crypto needs from Zarith.
• Make an implementation library mirage-crypto-bigint.zarith that just passes through to Zarith.

With a virtual library we could use the slower but much more portable num to implement the virtual library (ie. mirage-crypto-bigint.num). That would allow a slow Windows implementation. That slowness would be fine for many use cases (ex. testing out a web server on a Windows desktop) since I think the common use case is to deploy TLS endpoints on Linux. I believe TLS is the main use case for mirage-crypto-pk.

Someone would also be able to implement a new constant-time bignum implementation. I might even port a bignum from Rust or bind another C bignum library myself eventually, but that is a small chance since for most of my needs I prefer TLS offloading. (Yet if I had to use in-process TLS I would avoid the GMP library both because of its license and how difficult GNU software is to build with the MSVC compiler)