Sqlx postgres support

[sergiomeneses]

closes #7 .

[paulgb]

Thanks for this, @sergiomeneses!

I've opened #9 as an alternative approach to this.

The advantages, as I see it, are:

• This should work for all sqlx backends, without requiring us to create/maintain feature flags for each.
• It should be future-proof for new major versions of sqlx, without requiring that we update fractional_index to point to the same dependency version.

The disadvantages are:

• It requires an extra incantation when deriving AsRow
• It doesn't work for Option<FractionalIndex>, though you can use it with a nullable column (it will return the default FractionalIndex when the column is null).

On net, I think the advantages outweigh the disadvantages, but I'm open to thoughts here.