A bit of a big commit because I wanted sqlite queries to run
async and there isn't a sane way to tell in mlua whether a
given context is safe to yield for an await, which meant making
every case where we call into lua be fully async.
This adds a simple but powerful binding to sqlite that performs
its IO in a blocking tokio thread pool, and returns the results
back to lua.
There is no implicit caching of queries, and no implicit connection
pooling.
Wez Furlong