Unlimitted multi-mapping

Please find my implementation of unlimited multi-mapping.
There's a bug that I need to trace in the Query<> version that I need to track down, but the multiple-result version works fine.

Please see the test for how I'm employing this method.

This all uses the Dapper MultiMap internals and therefore shouldn't exhibit any perf issues etc. However, I'd implore Sam & Marc if possible to look at this for me in case I've missed something glaringly obvious.

And may I express my thanks to the team (Marc, Sam etc.) for a life-changing (yes!) open source contribution.

Mal