[PATCH 6 of 6 RFC] radixlink: use C radixlink get implementation if available

[Jun Wu]

Excerpts from Augie Fackler's message of 2017-05-24 17:26:05 -0400:
> Overall it sounds like this is worth doing, especially since it sounds
> like it could also be used for things like speeding up node->rev
> conversions later on.
> 
> I didn't go over the specifics of the implementation /too/ carefully,
> but it looked reasonable. Care to do a v2 with timing information
> added to this last commit?

Sure.

For other reviewers looking into details of the format: A known issue is
memory alignment is not taken care of so reading 4 bytes may be problematic
on some platforms. They will be addressed in V2.

> [...]