SI-7261 BooleanSetting to Boolean conversion

Add an implicit for BooleanSetting to Boolean, since while we never expect the Spanish Inquisition, we do expect a boolean in an if clause.

The first cut put the implicit at AbsScalaSettings, but the more general case is at reflect/internal's MutableSettings.  There was no particular desire to privilege mutability.

There is an additional implicit to convert BooleanSetting to BooleanFlag, in the second commit.  Maybe you knew there was a BooleanFlag.

The second commit tries to be minimalist, just to 6x the `.value`.  There are two or three spots where an expression has been restructured, but no intentional changes in behavior.

Review by @paulp who liked the idea and @retronym who has already egged on.