Remove algebra.number

[tixxit]

There was some discussion in gitter, but the general gist is that

• algebird and cats both don't use any of the type classes in algebra.number
• spire has since expanded on these type classes to make them more useful, but
• the type classes in spire require number types from Spire and cannot be ported over

I think the best bet for now is to remove the type classes. If we find we want some parts of them later on, we should think of a way of pulling them back out without disrupting the utility of Spire's versions.

[rklaehn]

:+1:

Should we do this before or after the initial "spire goes algebra" release?

[non]

:+1: I think we should do this before.

[johnynek]

Looks good.