parsegen: unify the "next" frame to go onto stack.

parsegen: unify the "next" frame to go onto stack.

We current have a current 'state' in the parser and a 'sym'
which is a local variable passed around in different ways.
Both of these get pushed onto the stack at the next 'shift'.

We will shortly add some more fields to the stack frame, so unify
'state' and 'sym' in to a 'next' struct in the parser struct which can
easily be extended.

Signed-off-by: NeilBrown <neilb@suse.de>