Simplify equality reducer

[njordhov]

The value-tuple state in flip-coin.clar can be reused and just passed along until and after a match, simplifying the implementation and making it more efficient:

clarity-smart-contracts/contracts/experiments/flip-coin.clar

Lines 16 to 23 in fec22ba

	(define-private (is-even (item (buff 1)) (value-tuple {value: (buff 1), result: (buff 1)}))
	(let ((val (get value value-tuple)))
	(if (is-eq item val )
	{value: val, result: 0x00}
	{value: val, result: (get result value-tuple)}
	)
	)
	)

=>

(define-private (is-even (item (buff 1)) (state {value: (buff 1), result: (buff 1)})) 
   (let ((val (get value state))) 
     (if (is-eq item val) 
       {value: val, result: 0x00} 
       state))) 

Also, consider turning result into a boolean initially set to false to eliminate the test on the result of the reduction:

(define-read-only (even (value (buff 1)))
  (get result (fold is-even even-buffer {value: value, result: false})))