move get_arity to CaseLamb
move some functionality from W_Closure to CaseLambda
move W_Object to its own file
oops, don't jit the plausibility check
make closures with one case their own environment

only works for case-lambdas with one case (=regular lambdas). Reduces
the amount of pointer chasing done.
obscure: start jitting from closure creation

this is mostly motivated by church numerals. There the unwinding of the
call stack and creation of closures for n->f was not jitted at all.

this might be solved by a more general form of reduce_prim_cont.