Usability improvements for snarl size limit

[faithokamoto]

If a snarl has more nodes than some pre-set limit (by default 50000, see vg PR), then instead of storing all pairwise distances, it will store only distances to its boundaries. This is called an "oversize snarl". I present for you three usability improvements:

• The temporary distance index now has a field to remember its largest snarl's size, which can later be reported to the user so they know how far up they would have to adjust the size limit.
• If a snarl's size is equal to the limit, it is now not an oversized snarl. This was what the documentation claimed but the code failed to back it up.
• A convenience function is_oversized_snarl() which checks if a snarl is oversized or not; currently only used in a test case.

[faithokamoto]

I now wonder if maybe I shouldn't have merged master into this branch if Xian isn't ready to release that other stuff yet.