std::variant: Fix use of incompatible api

Refactors any uses of the mapbox variant specific api and converts them
to an api compatible with std::variant and mapbox variant.

Tested:
    Built and run through the unit test suite.

Change-Id: Ie9b83fd638c495859fe98b5de86d9d3c7c1a27af
Signed-off-by: William A. Kennington III <wak@google.com>
3 files changed