Add "almost unsigned" integer types

[kriswuollett]

As mentioned in #856, I encountered a runtime issue when testing a query that used an u64 on SQLite. While that is not likely to change, perhaps there can be some fully supported Newtypes for "u7", "u31", and "u63" so that developers can maximize their range of desired ints, and just return an Err from a try_into when building a query.

I realize this is something I / everyone could do themselves, but seems like it could possibly be widely used so best if distributed as part of SeaQL itself?

[kriswuollett]

On the other hand, this is probably best provided by a third-party library since this concept could be requested to cover many other types from third-party crates.