[bug] field shift op bug

[Dreamerryao]

In the shift operation, bit length is compared to partition, but this is not reasonable.
For example, let p = 17, a = 5 and b = 6. Then run the following code, which will shift the 5 six bits to the left, but we're still looking for a rightward shift.
Therefore, we should use p/2 as our criterion, as described in the circom documentation.

Existing implementation:

shr(a, b) {
        if (b.lt(this.bitLength)) {
            return a.shiftRight(b);
        } else {
            const nb = this.p.minus(b);
            if (nb.lt(this.bitLength)) {
                return this.shl(a, nb);
            } else {
                return bigInt.zero;
            }
        }
    }