Mark btree_gist functions as PARALLEL SAFE

Hi,

On the back of this thread[1] over at pgsql-general, I've attached a patch that marks the functions in btree_gist as PARALLEL SAFE.
This is primarily to allow parallel plans to be considered when btree_gist's <-> operator is used in any context; for example in an expression
that will be evaluated at execution time, or in a functional column in btree or gist indexes.

In the latter example, despite the functions already being marked IMMUTABLE, attempts to retrieve precomputed values from a functional index during an index scan or index-only scan still require the function to be marked PARALLEL SAFE to prevent dropping down to a serial plan.

It requires btree_gist's version to be bumped to 1.6.

In line with this commit[2], and for the same reasons, all functions defined by btree_gist are being marked as safe:

---

"... Note that some of the markings added by this commit don't have any
effect; for example, gseg_picksplit() isn't likely to be mentioned
explicitly in a query and therefore it's parallel-safety marking will
never be consulted. But this commit just marks everything for
consistency: if it were somehow used in a query, that would be fine as
far as parallel query is concerned, since it does not consult any
backend-private state, attempt to write data, etc."

---

I haven't added any more tests, but neither did I find any added with the above commit.

"CREATE EXTENSION btree_gist" runs successfully, as does "make check-world".

This is the first patch I've submitted, so if I've omitted something then please let me know.
Thanks for your time,
Steven.

[1] https://www.postgresql.org/message-id/DB7PR09MB2537E18FF90C1C1BBF49D628FD830%40DB7PR09MB2537.eurprd09.prod.outlook.com
[2] https://github.com/postgres/postgres/commit/2910fc8239fa501b662c5459d7ba16a4bc35e7e8

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