Implement and test default string formatting in JVM and JS

Relax Duration.toString tests for JS and Native rounding specifics

libraries/stdlib/jvm/src/kotlin/time/formatToDecimals.kt

@@ -5,7 +5,32 @@
 
 package kotlin.time
 
+import java.math.RoundingMode
+import java.text.DecimalFormat
+import java.text.DecimalFormatSymbols
 import java.util.*
 
-internal actual fun formatToDecimals(value: Double, decimals: Int, unitName: String): String =
-    String.format(Locale.ROOT, "%.${decimals}f%s", value, unitName)
+private val rootFormatSymbols = DecimalFormatSymbols(Locale.ROOT).apply { exponentSeparator = "e" }
+private val precisionFormats = Array<DecimalFormat?>(4) { null }
+
+private fun createFormatForDecimals(decimals: Int) = DecimalFormat("0", rootFormatSymbols).apply {
+    if (decimals > 0) minimumFractionDigits = decimals
+    roundingMode = RoundingMode.HALF_UP
+}
+
+internal actual fun formatToExactDecimals(value: Double, decimals: Int): String {
+    val format = if (decimals < precisionFormats.size) {
+        precisionFormats[decimals] ?: createFormatForDecimals(decimals).also { precisionFormats[decimals] = it }
+    } else
+        createFormatForDecimals(decimals)
+    return format.format(value)
+}
+
+internal actual fun formatUpToDecimals(value: Double, decimals: Int): String =
+    createFormatForDecimals(0)
+        .apply { maximumFractionDigits = decimals }
+        .format(value)
+
+private val scientificFormat = DecimalFormat("0E0", rootFormatSymbols).apply { maximumFractionDigits = 2 }
+internal actual fun formatScientific(value: Double): String =
+    scientificFormat.format(value)