npm / arborist / #259

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use a linter

We have more people contributing to this library now, so adding some
automated consistency is looking more and more important.

Also, this found a few unused variables, which highlighted a bug, albeit
a harmless one: a timer string was being created, but then no timer
being attached to Shrinkwrap loading.  (That's why the reify snapshot is
updated.)  Some other light refactoring was done in a few places to get
eslint to produce a reasonably readable output, and I think mostly for
the improvement of readability.

I don't 100% agree with every choice the linter is making here.
Specifically, I prefer using {} around single-line blocks in some cases,
especially loops, but they are omitted most of the time, so that's the
way I set the config.  Consistency is good though, and this will help us
maintain it.

Linting is set up as a posttest operation, so we can catch things
without having it stubbornly refuse to even run tests until it's all
sparkling.

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