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Stiff stepper Rcpp::as tester.
Track stiff stepper categories
Stiff system adaptive integration for all three targets
Complete rewrite of how steppers are handled.

Stiff and nonstiff steppers (i.e., internal state) are both handled
in same object type.  Collection is done with boost::any rather than
boost::variant, though we have to keep a copy of that header in the
package now.

Runtime dispatch is done with a big ugly series of switch statements,
but they can actually be changed to a different approach relatively
easily.

Code generation with cog to remove some of the error-prone duplication.