parsegen: detect left-recursive symbols in non-final position.

A left-recursive symbol that appear other than at the end of a
production causes problem for indent-based parsing, as describe in the
document.  So teach parsergen to be able to report them.

Ocean currently has several of these, which I'll need to look into at a
later date.

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