Save start, stop, and step as properties of range

[parzhitsky]

In case of numeric implementation, save start, stop, and step as properties of range:

const { start, stop, step } = xrange(10);

start; // 0
stop; // 10
step; // 1

const { start, stop, step } = xrange(2, -3);

start; // 2
stop; // -3
step; // -1

const { start, stop, step } = xrange(42, 17, 5);

start; // 17
stop; // 42
step; // 5

const range = xrange(7, () => true, Math.random);
const { start, stop, step } = range;

start; // undefined
stop; // undefined
step; // undefined

"stop" in range; // true
"step" in range; // true

[parzhitsky]

This requires implementing a wrapper around core (@xrange/core) or reimplementing core to plain (although iterator-protocol-compliant) object, rather than a generator.

[parzhitsky]

Blocked by parzh/xrange__core#13

[parzhitsky]

start of a functional iterator is changed to undefined to address #48