parsergen: allow prefix code for do_reduce to be provided.

parsergen: allow prefix code for do_reduce to be provided.

This allow common variables to be declared and initialised
for all reduce fragments.

Use this in oceani to declare 'c' holding the parser context,
so we don't need to call config2context in so many places.

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