Possible pluggable C extension for performance - #1066

I'm creating this PR more as a start to a conversation as opposed to a request that I expect to be accepted any time soon. We like being able to use PyMySQL because of the ease of installation and the fact that it can be used without any system-level libraries being installed. However, the performance leaves a lot to be desired when dealing with larger result sets.

After some investigation, it looked like most of the time was being spent in the row data fetching and data conversion, which all stems from one method in the connection (`read_rowdata_packet'). An idea came to mind to replace that method (and anything that it calls) with a C extension to see how much performance could be increased. It ended up working much better than expected and the performance improvements actually made the client (arguably) the fastest MySQL Python client available (arguably because the Mariadb client is pretty fast too). It is definitely the fastest Python client that doesn't require any external libraries to run.

I thought I'd bring the work up to your team to see if you might want to collaborate on it as a possible plugin for PyMySQL, or to see if it should remain a separate project.