Universalize all "typeclass traits" in Spire.

None of our typeclasses should be doing work in constructors,
defining fields, etc. This commit adds "extends Any" to all
of these classes, making them eligible to be extended by
value classes.

Some other traits were left alone (notably traits in spire.random).
It may be worth updating those too, but I think we'll need to
look at that case a bit more closely.

Review by @tixxit or @larsrh, and @paulp if he wants.

This PR resolves #318.