Remove spire.implicits._ imports from main code.

This will hopefully help things compile faster (or avoid
recompilations) but also makes it easier to be clear about
which code depends on which code.

The wildcard imports from spire.implicits._ and spire.math._
are really useful when authoring code, but are usually
much broader than actually needed.
Reduce spire.math._ imports.

Since spire.math is such a big package, this change makes
it easier to be clear on what is actually being used.
Especially with methods like max (which are provided by
scala.math, spire.math, and also many types) it can be
confusing to use a wildcard import.

Hopefully this will also make recompilation a bit easier.
Reduce spire.algebra._ imports.

Like the preceeding commits, this is designed to try to
limit visibility a bit. It's probably less essential
than the other two commits, since spire.algebra doesn't
contain any implicit conversions or important methods.

I've left wildcard imports in places where a file needs
more than 6-7 types from the package.