Depend on non/algebra instead of non/spire

[rklaehn]

It seems that the main reason for the spire dependency is to provide basic typeclass instances so things work out of the box. None of the advanced features of spire are being used.

But spire is quite a heavy dependency, which prevents people from using debox. See https://twitter.com/li_haoyi/status/677986085360689153

I would not want to have the typeclass instances in a separate library. But once spire depends on algebra, debox would only have to depend on algebra? That would reduce the size of the dependencies to something more reasonable.

If the required typeclass instances end up in cats-kernel or whatever, that would be fine too.