[FRONTEND] Refactor jit.py. (#2556)

[FRONTEND] Refactor jit.py.

The goal is to simplify the code and make it more flexible before we
change the kernel launch syntax to
`kernel[grid, compiler_flags(...)](...)`.

The main changes here are:

 - Get rid of the eval'ed code in make_launcher.  We can do everything
   using bind().
 - Add KernelParam and KernelArg classes, letting us get rid of the
   parallel arrays/dicts indexed by parameter index.
 - Get rid of duplicated kernel launch code in the cache-hit/cache-miss
   branches.