Install local assertions into the blossom algorithm

[ecpeterson]

In tracing through the algorithm as part of an earlier internal PR, @karalekas suggested a couple of assertions that could be made while respecting memory locality:

• When installing a master edge, check that the source node of the proposed edge matches your address.
• When substituting one address for another over a collection of slave targets, check that exactly one such substitution occurs.

There are probably many more assertions that could be introduced which would not violate locality or consume too much runtime—but, in my experience, they are hard to list off without reading through the code line-by-line. This issue is an instruction to take the time to do exactly that.

[ecpeterson]

See also #17.