Restore binary compatibility with 1.14.0

[smarter]

dbaeb80 broke binary compatibility by
making BooleanOperators private, this does not match the plan discussed
in #540:

The 1.15.x series will preserve binary-compatibility with 1.14.0 but
is expected to break source-compatibility (e.g. removing the implicit
keyword from some definitions)

This commit implements that: BooleanOperators is public again but is
also made non-implicit.

[ashawley]

I wrote that I had made the change in #498 (comment). I believe there was a reason for it rather than just removing the implicit. I'll need to try and remember why I did this. I admit that my comment was unfortunately very cryptic.

The BooleanOperators is now private[this], for backwards compatibility, in favor of importing the implicit propBoolean instead.

[non]

@ashawley OK to merge this?

[non]

I'm just gonna go ahead and merge this. Thanks again @smarter for catching this!

[ashawley]

I remember the motivation now for changing BooleanOperators to private[this]: It was for usability around error-messages v. deprecation messages.

[ashawley]

Further, changing the implicit to package-private actually is binary compatible. Mima is flagging it as a binary compatible change, but that is only for Java callers, see lightbend-labs/mima#375.

It's good that Mima does this for those occasional Scala libraries with downstream Java users. I'm pretty confident we don't have any Java callers, so this Mima check could be ignored.

[smarter]

Further, changing the implicit to package-private actually is binary compatible.

Maybe, but before this PR, private[this] was used which does get emitted as private and isn't binary compatible.

[ashawley]

I believe that would emit the same byte code as the example in lightbend-labs/mima#375, but I could re-check.

[smarter]

It doesn't:

package bla

class A {
  private[this] def foo: Int = 1
  private[bla] def bar: Int = 1
}

% scalac A.scala
% javap -p bla/A.class
Compiled from "pp.scala"
public class bla.A {
  private int foo();
  public int bar();
  public bla.A();
}

[ashawley]

So it does. I guess I made the incorrect change, then. I wrongly assumed private[this] was like package-private and therefore binary compatible. The change should have been private[scalacheck]. Thanks.