Resurrecting the MySQL backend

[LaurentRDC]

MySQL remains a serious choice over Postgres in many domains. We should support it.
There exists a beam-mysql, but it is unmaintained. beam-mysql is based on mysql, which is also unmaintained.

We should:

• import beam-mysql in this repo;
• Switch over to use mysql-haskell, which is maintained

I'm a little oversubscribed at the moment, but I'm happy to review pull requests!

[LaurentRDC]

Requires winterland1989/mysql-haskell#86