Require visibility for class constants

[mbabker]

With PHP 7.2 being required going forward, I propose all class constants are required to have a visibility declared so it is explicitly clear what constants are visible in what scopes. Obviously, most constants will have to be public for B/C.

[wilsonge]

Makes sense to me

[TomasVotruba]

In case you plan to add private/protected modifier in next version with BC break, here is a way to automate it:https://tomasvotruba.com/blog/how-to-add-visbility-to-338-class-constants-in-25-seconds