Type roundtrip tests (#1185)

A new set of tests to simply check that we can "roundtrip" a Datum.  That is, we return the same value as we were given in a `#[pg_extern]` function argument.

Essentially this tests that the type's `FromDatum` and `IntoDatum` impls match, and in the case of arrays, tests that the iterator impl for `Array<T>` works for its individual datums.

In doing so, this uncovered a bug with converting a `pg_sys::Point` (and `pg_sys::BOX`) into a Datum, so those are fixed in here as well.

The addition of `#[derive(Copy)]` to the "Complex" type in pgrx-tests is just to make it work well with the roundtrip tests here.

Author: eeeebbbbrrrr

pgrx-tests/src/tests/mod.rs

@@ -42,6 +42,7 @@ mod pgrx_module_qualification;
 mod postgres_type_tests;
 mod range_tests;
 mod result_tests;
+mod roundtrip_tests;
 mod schema_tests;
 mod shmem_tests;
 mod spi_tests;