Improve performances of BigInteger.numberOfDecimalDigits().

[glureau]

Similar PR than previous one but with focus on the BigInteger.

    @Test
    fun perfTest() {
        val bd = BigInteger.parseString("123")
        val duration = measureTime {
            repeat(1_000_000) { bd.numberOfDecimalDigits() }
        }
        println("Duration: $duration")
    }

Result (best of 3 runs)

platform	Before	After	Time reduction
iosSimulator	4.60s	0.62s	reduction of 86%
JVM	219ms	75.2ms	reduction of 66%
macosArm64	4.91s	0.67ms	reduction of 86%

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All benchmark data

All runs are done on the same MacBook Pro M3 with 36GB RAM.

Before

• ios 4.83, 4.60, 4.67
• jvm 234ms, 219ms, 241ms
• mac 4.92, 4.91, 4.98

After

• ios 622ms, 617ms, 619ms
• jvm 11.6ms, 9.8ms, 11.4ms
• mac 673ms, 666ms, 666ms

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Just a side note, I took a relatively short value for the benchmark, I guess performances may vary with length (like the previous implementation). I only run one more test on JVM with "123456789012345678901234567890123456", before 232ms, after 54ms, so I guess we're still in a better place.