nls: add compose sequences for leftward, upward and downward double arrow

These sequences follow the logic of: the arrow shaft (=) first, the arrow head (< or > or ^ or v) second. This is so because the sequence <= is already taken for ≤, and there is no obvious ASCII character for a double vertical line, so we have to make do with a horizontal one.

Since commit 6101b967 from eight years ago, there is a compose sequence for the rightward double arrow, but not for the other three directions.

This fixes issue #138 (closed).

Requested-by: Mélanie Chauvel

Signed-off-by: Benno Schulenberg bensberg@telfort.nl