Rewrite cpu_switch(). The most notable change is the fact that we now

Rewrite cpu_switch(). The most notable change is the fact that we now
have f16-f31 as part of the context. The PCB has been reorganized to
better match how we save and restore the (preserved) registers. This
commit also moves the context restoriation to its own function (named
pcb_restore), as we did with pcb_save.

Only minimal effort has been put in writing optimal assembly. The
expectation is that there will be more rounds of changes.