Begin clean up of vm.c
Begin clean up of vm.c

Replace the oparg macro with 5 named virtual registers, combine
pc++ with vm_next() macro to be more terse, and move setup and
teardown logic of janet_continue into a separate function.

These changes are preparation for using setjmp/longjmp to do
error handling in the VM. Introducing longjmp for error handling in
the VM would allow it to be used in the C API, which could result in
simpler, more compact code.

bakpakin/gst,bakpakin/gst