Implement a decode/issue FSM between fetch and execute

Implement a decode/issue FSM between fetch and execute

The idea is for it to:
* keep looping "fetch" while VL==0 on a vector instruction.
* keep looping "execute" while SRCSTEP != VL-1.
* unless PC/SVSTATE was modified by "execute", in that case do go back
to "fetch".
* update PC and SRCSTEP accordingly.