make PropertyType public, add Path factory method

[albertchen-sifive]

Contributor Checklist

• Did you add Scaladoc to every public function/method?
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Type of Improvement

• Feature (or new API)
  This makes PropertyType and its companion object public and adds a factory method to construct a Path from an IsMember target.

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• Squash: The PR will be squashed and merged (choose this if you have no preference).

Release Notes

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[jackkoenig]

If we need to make the object public, then we need to make a bunch of its contents package private, eg. def makeSimple which otherwise is a huge internal API exposed.

[albertchen-sifive]

actually we don't need this to be public. I reverted this change

[jackkoenig]

LGTM!