- We now go ahead and deconstruct the range value in the `FromDatum` implementation.  Immediately, this fixes issues with `pg_sys::RangeBound.val` datums potentially being able to be used after free.

- `RangeData` is gone and replaced with a more idiomatic `RangeBound` enum, to which the internal Postgres `pg_sys::RangeBound` struct can be mapped.

- We provide `From<std::ops::Range*>` impls for all of Rust's built-in Range types.

- Our `pgx::Range` can be constructed from any type that can be converted into a `RangeBound` and we provide direct impls for the set of supported `RangeSubType` types along with `Option<T: RangeSubType>` which maps `Some` to `::Inclusive` and `None` to `::Infinite`.

- The idea that a Postgres range can be "empty" is properly relayed through `Range`'s public API

- add an example (albeit puny)

…c cleanup

- add a few help methods to `RangeBound`
- `impl Display for Range<T> where T: Display`