Support PG ranges

[sgrif]

The SQL type should be Range<ST: CanAppearInRange>. We should provide an impl to/from SQL for (Bound<T>, Bound<T>) where T: FromSql<ST, Pg>. Bound is currently unstable, so we'll need to stick this behind the unstable feature. It appears to be on track for stabilization soon though, so I don't think we need to support any stable types.

For this to work with codegen we'll also need the following:

pub type Int4range = Range<Int4>;
pub type Int8range = Range<Int8>;
pub type Numrange = Range<Numeric>;
pub type Tsrange = Range<Timestamp>;
pub type Tstzrange = Range<Timestamptz>;
pub type Daterange = Range<Date>;

The impls for serialization/deserialization look pretty straightforward. Format appears to be a one byte set of flags (inclusive vs exclusive, whether start or end are unbounded), 4 byte length for upper value followed by data if present, 4 byte length for lower value followed by data if present.

[sgrif]

I wouldn't quite call this easy for newcomers, but it is at least fairly isolated.

[Eijebong]

I'm working on that now, I just have some questions:

• How do we treat infinity ?
• OIDs ? Apparently each range has a different OID and arrays of ranges too. Should I request a metadata thing in the CanAppearInRange trait ?

[Eijebong]

Fixed by #1021