SI-7261 Implicit conversion of BooleanSetting to Boolean and BooleanFlag

This allows a more compact expression `if (settings.debug)` instead of
`if (settings.debug.value)`, in order to improve readability.

The first commit contains the two implicit definitions, and the second
commit deletes dot value where the expected type is Boolean, viz., in
conditionals or, rarely, function calls.

This is a more conservative version of https://github.com/scala/scala/pull/2297.

Review by @paulp who is still trying to conserve the code base