[1.0][OrderedSet] Make _checkInvariants public and call it in more operations

[lorentey]

It seems useful to be able to manually call _checkInvariants while debugging a package issue. This is public as in public, not public as in Public API -- it's underscored.

This also adds more _checkInvariants invocations in functions that return newly created ordered sets.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[lorentey]

@swift-ci test

[lorentey]

(The idea here is that this may allow us to find the culprit in #104 quicker, if it proves to be a bug in the package.)

[glessard]

Looks quite reasonable.

[lorentey]

@swift-ci test

[lorentey]

@swift-ci test