Configuration loading and typed accessors.

Reads `resources/conf/<env>/config.edn` through Aero and exposes the
settings libraries need. Assembling an Integrant system from those settings
is the application's job — see `wagoe.system-config` in the monorepo, or the
`config.clj` that `wagoe new` generates.

This lives in a library because four published libraries used to reach for
it with `requiring-resolve`, which worked in the monorepo and in generated
projects and nowhere else — and `wagoe.user`'s CLI resolved a `db-spec` that
generated projects never had (BOU-306).

Usage:
  (def config (load-config))
  (db-spec config)