Make InferWidths thread safe (#1775)

Change the class-global, but private ConstraintSolver object inside
InferWidths to instead be constructed on each execute invocation. This
prevents issues with thread safety where running the same InferWidths
object at the same time would cause the ConstraintSolver to get
trampled on.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>