scheme: Reduce core iterators requirement to R6RS

[okuoku]

Currently, (yuni scheme) is vague that we'd follow R6RS or R7RS(SRFI-1). Follow R6RS semantics this time; iterators that take any sequence as argument, its length must be equal. Implementations still allowed to ignore the rest.

Users can explicitly request SRFI-1 to get R7RS behaviour (with external library.)