perf: optimize IF THEN ELSE for cases where condition expression resolves to a constant

[chrispcampbell]

Fixes #102

See issue for more details. I updated the existing prune test (which is used to verify the unused reference elimination code) with new cases to cover the IF THEN ELSE optimizations.

At some point I'd like to reimplement the equation reader phase to produce objects that are AST-like, and then have the code gen phase use those objects instead of revisiting the whole model structure. That way optimizations like this and the dead code elimination (and others we could do, like constant folding) could be expressed as AST transforms. But that's for another day (probably way down the road). For now, I think this gets the job done, and wasn't too terribly invasive.

[ToddFincannon]

That's a surprisingly effective optimization. Because ExprReader is specifically oriented to analyzing constants, I might have named it something less general. But it also makes sense to keep expanding its capability as necessary under the more general name.

[chrispcampbell]

That's a surprisingly effective optimization. Because ExprReader is specifically oriented to analyzing constants, I might have named it something less general. But it also makes sense to keep expanding its capability as necessary under the more general name.

That was my thinking as well, so I left it with the more general name. Easy to change later if we find a need for different kinds of expression readers.