Changes

Introduce PairValueIndex and AttachIndices

There are two use cases that are currently mixed when using
LegacyValues:

1) Legacy classes use LegacyValues to pass to the new interface
2) User wants to attach indices to the passed primitives without
   constructing them themselves

The difference is that 1) should not be exposed to a user, while 2) is
intended to.

With the introduction of the RangeTraits in the future, the use cases
diverge: 1) will still use AccessTraits for backwards compatibility,
while 2) will switch to using RangeTraits.

This patch separates the two. They still share a lot of commonalities at
the moment, but that will change with RangeTraits.

In addition, the order is swapped between value and index in the struct.
The reason being is that it is easer to align an index (likely 4-byte
int) than a value (which could be a 12-byte point, 24-byte box, or
whatever).

Fix MLS

Move AttachIndices to Experimental and add attach_indices