Stateless feature probing

[CAD97]

Alternative to #35. cc #24, #28.

This API puts the burden onto the user to know how to best check that a feature works as expected (whatever statements are used to check it) rather than reusing existing probe_/emit_ methods. This means that probe_features is a bit harder to use, but the developer testing for the feature(s) almost certainly already knows what they want to do with the feature well enough to write a simple check.