Generate case expression for ifs.

Instead of just calling ite. Perhaps a tiny performance improvement. Perhaps
not.
Manually inline the top level call to casef in codegen for case.

But preserve sharing while donig so. Improves shampi performance by 20%.
Move casef definition into symbolic branch.

No performance improvement, but I think it's cleaner.
Specialize for concrete evaluation without exponential blowup.
NOINLINE `primitive'.
Added to todo list for supporting -O with -fplugin.
Make int_leq primitive, not int_compare.

To be more consistent with integer_leq, and to avoid a primitive returning the
Ordering type.
Resurrected the symbolic model checker implementation.
Remove Prim constructors.

They currently aren't used, and if we ever would have used them, I suspect it
would not have helped performance much, and they aren't needed for
correctness.