Redefine XRange as `IterableIterator<number>` instead of a generator

[parzhitsky]

Change definition of XRange type to IterableIterator<number> both in xrange and @xrange/core packages:

• https://github.com/parzh/xrange/blob/develop/src/xrange.ts#L9
• https://github.com/parzh/xrange/blob/develop/src/index.ts#L6
• not this: https://github.com/parzh/xrange/blob/develop/src/typings/xrange.ts#L7 (already deprecated)
• https://github.com/parzh/xrange__core/blob/master/src/xrange.ts#L1
• de-diversify definitions of XRange (define it exactly once in @xrange/core)