Implement Fiber.reset

[Goju-Ryu]

When reading the concurrency section on fibers I had some trouble understanding why fiber.reset() and fiber.status() was called in example only defining a fiber called task. After reading through some documentation and testing in the playground I have come to the conclusion that it is probably an error of the documentation.

The examples in question are the following:

cyber/docs/docs.md

Lines 2364 to 2402 in f3bda25

	### Reset state.
	To reset a fiber to its initial state, invoke `reset()`. *Planned Feature*
	When reset, the existing stack is unwinded, the program counter returns to the starting point, and the state is set to `.init`:
	```cy
	func fib(n int) int:
	coyield n
	if n < 2:
	return n
	return fib(n - 1) + fib(n - 2)
	var task = coinit(fib, 10)
	-- Progress the fiber...
	print(coresume task) -- Prints "10"
	print(coresume task) -- Prints "9"
	print(coresume task) -- Prints "8"
	-- Reset back to the start with the `.init` state.
	fiber.reset()
	print(coresume task) -- Prints "10"
	```
	### Rebinding arguments.
	Arguments attached to the fiber can be rebinded with a different set of values. *Planned Feature*
	This allows fiber reuse, instead of creating a new fiber:
	```cy
	var task = coinit(fib, 10)
	-- Run task to completion.
	var res = 0
	while fiber.status() != .done:
	res = coresume fiber
	print res
	fiber.reset()
	fiber.bindArgs(20)
	-- Run task again with the new argument...
	```

[fubark]

Yep, that's a typo. Also, Fiber.reset hasn't been implemented.