Add skippable defaults

[mdekstrand]

This adds a new flag, skipIfUnusable, to default specifications that instructs Grapht to skip that default binding if one of its direct dependencies cannot be resolved. If the direct dependencies can be resolved, but one of their dependencies fails, the injection fails as if the skippable default were not skippable. Skippable defaults will usually fall back to a null.

There are some limitations so far:

• This plays very badly with deferred dependencies.
• It does not look for other bindings if the skippable binding cannot be used; rather, it just backtracks to the previous binding. Also, it just backtracks once, so if you have a binding to a non-instantiable interface, which is then defaulted with a skippable binding, the skip won't work.
• The behavior on transitive dependency failures could perhaps use review — do we want transitive dependency failures to cause a skip rather than a failure?

Includes pretty full tests, except for the skip flag appearing in META-INF defaults.

The big advantage of this: we can reduce some cases where we were using providers that return null in LensKit. Providers returning null is an anti-pattern IMO.

[mdekstrand]

Since this works and fixes the problem I need it to in LensKit, I am going to go ahead and merge. The deficiencies primarily arise in pathological edge cases, and we can improve the error reporting and some linting. Also, I have written it to be conservative and fail more; design changes are most likely to decrease the failure modes, which is a compatible change.