BigDecimal: roundSignificand perf improvement (when using scale)

use isZero instead of comparing with BigInteger.ZERO (I forgot this one on the other PRs...)
skip the extendedSignificand when using scale.
[!Important] I understand that when using scale, BigDecimal(BigInteger(123), exponent, decimalMode) == BigDecimal(BigInteger(12300000), exponent, decimalMode), no matter the exponent or decimalMode. If this is true, then we don't need to calculate the 'pow' and this PR is relevant. Please be aware here I'm not very experimented so that's a supposition.

Many methods are impacted by this method, as I don't have much time right now I've only experimented add with BigDecimals using scale (performance without scale would be roughly similar), but I guess many other methods may have roughly the same improvement.

Benchmark on JVM, best of 3 runs, M3 Pro 36GB, 10 million iterations, with a warmup loop for JVM (starttime and JIT excluded for a more stable duration).

Code to reproduce:

    @Test
    fun perfAdd1() {
        val a = BigDecimal.parseString("1.01").scale(2)
        val b = BigDecimal.parseString("2.22").scale(2)
        repeat(1_000_000) { // JVM Warmup
            a.add(b)
        }
        val duration = measureTime {
            repeat(10_000_000) {
                a.add(b)
            }
        }
        println("Duration: $duration")
    }

Method	BEFORE	AFTER	% time reduction
add	12.0	11.4	5%

Important note here, this is computed on main branch as the other PRs are not yet validated. On my fork, I've merged them all on a temporary branch and re-run the same benchmark. Savings are way better and then this optimisation is saving 45% of the time when using the scale.

Method	BEFORE	AFTER	% time reduction
add	2.17	1.19	45%

Eventually with the 5 PRs we're passing from 12s to 1.19s, so a total 90% time reduction 🎉

Raw benchmark data

Before
Duration: 12.197146042s
Duration: 12.023203292s
Duration: 12.470724917s
After
Duration: 11.589598666s
Duration: 11.617750s
Duration: 11.409660500s

(from fork branch with all improvements)

Before
Duration: 2.305555875s
Duration: 2.381971125s
Duration: 2.165184959s
After
Duration: 1.188598625s
Duration: 1.266141417s
Duration: 1.318251250s

ionspin / kotlin-multiplatform-bignum

BigDecimal: roundSignificand perf improvement (when using scale) #303