Warn if interpolator uses toString

[som-snytt]

Adds -Wtostring-interpolated which warns just for standard interpolators.

-Wconf:cat=w-flag-tostring-interpolated:e to boost.

Also restores edge case f"${ "hello" }%b" which is "true" for non-boolean arg. That became an error in previous refactor, possibly intentionally. (It is arguably an obscure feature of format.) (Edit: the C.I. error reminds me that the change was to make the expected type Boolean, so that conversions work normally. That is also edge casey. Maybe it can try boolean, null, and any? Edit: yes, try non-any first, then any if allowed.)

The "default" %s is improved so that the warning position is at the interpolated element in s"$x".

Follow-up to #8654

[som-snytt]

Previous feedback at wartremover is to allow at least CharSequence and perhaps primitives with "canonical" representations (Int, Char, but not Float, Double). What about usual formatting of Int such as 1,024?

Therefore, no accommodation is made yet here.

A future version can take a typeclass, implicit locale, etc.