parsergen: add more power to symbol references in generated code

As well as symbol references like "$2", you can now use references
with letters like "$Ss".  This will find the shortest symbol in the
production that contains all the given letters in the given order.
There must be a unique shortest symbol.

If that same symbol occurs multiple times, later instances can be given
with a numeric suffix such as "$Ss2".

Signed-off-by: NeilBrown <neil@brown.name>