SI-8092 Refactor f-interp

A denshish refactor makes the FormatInterpolator a nice bundle
that destructures its input and flattens out the classes to
give the code some elbow room. Everything shifts left.

The `checkType` method is refolded and renamed `pickAcceptable`.

An additional test case captures the leading edge test, that
a % should follow a hole, and which is the most basic
requirement.