clean up leave_scope()

clean up leave_scope()

Rather than having a whole bunch of casting macros like

    #define ARG0_SV  MUTABLE_SV(arg0.any_ptr)

just add extra pointer types to the ANY union and directly do

    arg0.any_sv

etc.

Also, rename arg0, arg1, arg2 to a0, a1, a2 and reverse their order;
previously if 3 args A,B,C were pushed onto the savestack, they were
popped as a0=C, a1=B, a2=A; now do it as a0=A, a1=B, a2=C. The previous
(rather odd) order was an artifact of the old way leave_scope() used to
pop its args.

There should be no functional changes.