Returns to previous behavior of enabling lazy=>1/0 when importing default

This moves the functionality from `build` back to `default`.
Importing `default` gives you
`has attr_name => value, lazy => 1/0`
if not specified, attributes are lazy by default.

If you also import `nonlazy`, attributes will be eagerly initialized
unless explicitly stated otherwise.
Renames t/build-lazy-arg.t -> t/lazy-nonlazy.t