1.4.4: Add reverse encoders for packed repeated fields.

[j6carey]

Like the Vector encoders but unlike the forward-order Foldable encoders they avoid the need to save elements to the stack before encoding them, instead immediately encoding the final element as the first step. However, the new encoders lack the special optimizations around size checks that are possible for the Vector encoders, which have advance knowledge of the number of elements.

This change also avoids compiler warnings in benchmark code.

NOTE: This change should not be merged until we confirm that it is useful downstream in proto3-suite. This note will be removed when that confirmation is obtained. Meanwhile please do not feel obliged to review this PR (though comments are welcome if you feel inspired).