Reduce API surface

In #382 I expressed concerns about adding a variant of the `validate` method that doesn't require a full refined type, because I feel the current API surface is already quite large and the library provides sometimes more than one way to do things (especially with the addition of companions for refined types, see #369). It would be nice if we could reduce the API surface and make it more approachable.

This issue is about collecting weak spots in the API and things we can improve.

Start by reading the concerns in #382 and the companion additions mentioned in #369, then inventory the public API for redundant or weak spots. This issue names no files or tests, and its completion criteria are limited to collecting weak spots and identifying improvements.

Written by the indexing model from the issue text.