Constant-time utilities.

Pull constant-time methods out to a separate header, add tests.

(Imported from upstream's 9a9b0c0401cae443f115ff19921d347b20aa396b and
27739e92659d38cdefa21e51b7f52b81a7ac3388)

Change-Id: Id570f5c531aca791112929e6258989f43c8a78d7
3 files changed