refactor and regularise label/statement grammar

refactor and regularise label/statement grammar

Refactoring of the grammar around statements.  New production <barestmt>
encompasses a statement without label.  It includes all statement types,
including declarations, with no unnecessary intermediate non-terminals.
It generates an op tree for the statement's content, with no leading
state op.  The <fullstmt> production has just one rule, consisting of
optional label followed by <barestmt>.  It puts a state op on the front
of the statement's content ops.

To support the regular statement op structure, the op sequence for for(;;)
loops no longer has a second state op between the initialisation and
the loop.  Instead, the unstack op type is slightly adapted to achieve
the stack clearing without a state op.

The newFOROP() constructor function no longer generates a state op,
that now being the job of the <fullstmt> production.  Consequently it
no longer takes a parameter stating what label is to go in the state op.
This brings it in line with the other op constructors.