A system for defining and maintaining a collection of health checks with
support for both realtime and background checks.

The `salutem.core` namespace is the public interface of the system and should
be used in preference of the contained namespaces.

`salutem` is somewhat inspired by [dropwizard-health](https://github.com/dropwizard/dropwizard-health)
which may provide additional insight into its design.

Provides constructors, predicates and evaluation functions for checks.

Provides an asynchronous maintenance pipeline for maintaining up-to-date
results for the checks in a registry.

Provides constructors, query functions and resolution functions for
registries.

Provides constructors and predicates for check results.

Provides time utilities for use in check definitions and the maintenance
pipeline.