WL#5861: Use single value list partitions as index

For a list partitioned table where all partitions have a single value, it is
possible to use the partitioning function as index.

Example:

CREATE TABLE t (a INT, b VARCHAR(63)...)
PARTITION BY LIST (a)
(PARTITION p0 VALUES IN (0),
 PARTITION p1 VALUES IN (1),
 PARTITION p2 VALUES IN (2));

INSERT INTO t VALUES (1, "row 1"), (0, "row 0")...;

when implemented this should avoid filesort, and use the index given by
partitioning instead:
EXPLAIN SELECT * FROM t ORDER BY a;

One could also remove the partitioning columns from the PK in the partitions
handlers, making the PK shorter (which is good for InnoDB) in this case.
Like:
CREATE TABLE t (a INT, b INT, c VARCHAR(64)... PRIMARY KEY(a, b))
PARTITION BY LIST (a)
(PARTITION p0 VALUES IN (0),
 PARTITION p1 VALUES IN (1),
 PARTITION p2 VALUES IN (2));
Would create the partitions handlers with the structure:
CREATE TABLE t (a INT, b INT, c VARCHAR(64)... PRIMARY KEY(b))
Note: in LIST COLUMNS partitioning; no partition column can have the same value
in two or more partitions!