`avg` cannot be used on SQLite

[sgrif]

The return type of avg for all integer types is Numeric, which is the arbitrary precision floating point type. SQLite has no concept of this type. We can't really have the return type of avg be dependent on the backend (since we don't know the backend at the point avg is called). So we probably need to add some "support" for Numeric on SQLite. We should never infer this from diesel print-schema or anything like that, but we do need to do impl HasSqlType<Numeric> for Sqlite, and provide a FromSql impl for something there.

The question is whether we should have Numeric pretend to be Text, and provide a FromSql impl for BigDecimal, or whether we should have it pretend to be Double and provide an impl for f64. I'm guessing it should be the latter, since the floating point precision issues will happen on the DB side, but I'd like to see a query that demonstrates that avg(int_column) will in fact be limited by 64 bit float precision on SQLite regardless of what we do on our end.