Things that the initial SQLite PR will not handle but need to be fixed

[sgrif]

• Anything that invokes as_query on a command is a syntax error, as SQLite has no returning clause. We need to make AsQuery generic over the backend and not implement it for SQLite.
• CLI is tightly coupled to PG. We need to not only support Sqlite but also figure out how to make this pluggable by third party crates that want to add additional adapters. Worst case this is a single line in our code and a #[cfg] attr for every third party crate that asks.
• Codegen is tightly coupled to PG. We need to not only support Sqlite but also figure out how to make this pluggable by third party crates that want to add additional adapters. Worst case this is a single line in our code and a #[cfg] attr for every third party crate that asks.
• Support for date, time, and timestamp
• Support for numeric Numeric is not distinguishable from double on SQLite.
• Support for IN statements. Unlike eq(any(array)), this will cause unbounded growth of prepared statements so we need to be careful.

[sgrif]

Everything here has been done except #184