Support for 64-bit integers

[mfp]

This PR adds support for 64-bit integer fields and param binding.
I was able to test signed integers (full range from -9223372036854775808 to 9223372036854775807), but not unsigned integers yet -- the code is very similar though, making it less of a leap of faith.

Even though they compile, I was not able to use the stress tests: they segfault in the unmodified tree as of 862877a

Credits

The work on this PR was performed in the context of Ahrefs open-source work day:
https://twitter.com/javierwchavarri/status/1466774912148910088

[paurkedal]

I agree this is a good addition (CC @tatchi who mentioned it in #61). I don't have write access to the branch of this PR, so I've pushed my suggestion to [my own 64bit-integers branch](https://github.com/paurkedal/ocaml-mariadb/commits/64bit-integers/]:

• I merged the current master branch (avoiding some conflict resolving of a rebase).
• My third commit allows some extra conversions, which for int may overflow.

The rationale for the possibly overflowing conversion of int is simplicity and backwards compatibility, see also my commit message. We could consider raising an exception in case of overflow.

If you want to add more commits, can you merge in (parts of) my branch first, or rebase in cherry pick?