Implement UNNEST in sqlite. Expose it via the postgres emulation.

[gorbak25]

This PR adds UNNEST which is an table valued function for zipping multiple Rc<Vec<Value>> arrays together. Arrays are passed to sqlite using it's pointer passing interface. The array type accepted by UNNEST is the same which is accepted by rarray in rusqlite.

This makes it possible to make very efficient bulk queries in sqlite. For ex. bulk insertion:

INSERT INTO foo (col1, col2, col3, col4) 
SELECT value0, value1, value2, value3 
FROM unnest($1, $2, $3, $4)

or selecting/deleting a list of objects in a single statement:

SELECT * FROM foo WHERE foo.id IN (SELECT value0 FROM unnest($1))

This is very powerful as most patterns of

for params in parameters:
     sql.query(..., params)

Can get converted to some sort of single statement using unnest.

I've exposed this feature on our Postgres emulation. Unfortunately due to some limitations array parameters need to be strongly typed and for now at most 10 array arguments per unnest call are supported. For now 4 array types are available via postgres - text[], blob[], int[] and float[].

To use UNNEST via postgres make an explicit type cast, for ex:

INSERT INTO foo (col1, col2, col3, col4) 
SELECT value0, value1, value2, value3 
FROM unnest(CAST($1 AS int[]), CAST($2 AS float[]), CAST($3 AS test[]), CAST($4 AS blob[]))

UNNEST is very efficient if there is significant network latency between the client and database.

[somtochiama]

LGTM 🚀 .