Add fromDouble to Field. Small ops reorg.

This adds fromDouble to field. There is a default implementation that
uses Field ops, though it should be overidden.

This also moves the ops classes out to their own file, to minimize other
files' interactions with macros.
Add fromDouble method to some Field instances.
Move std instances to own sub-package.

This moves the standard type class instances to their own subpackage,
spire.std. This also adds a new importable object, spire.ops, that will
just import the operator implicits. Also, spire.implicits._ will not
only import the operator implicits, but also all of the stnadard
instances.
Fix up remaining compile/test issues in refactor.