allow initializer for RANGE and CHOICE expressions in strategy patterns

... to start the search (improve-strategy) with a known good value

concrete syntax is up for debate, e.g.,

RANGE(3..29)  =>  RANGEINIT(10,3..29)
CHOICE(foo, bar) => CHOICEINIT(foo, foo, bar)

extension: when executing a proof search, matchbox can read such an pattern, and uses just the initalizer.