[Wasm] Improve string constructor in stdlib

libraries/stdlib/wasm/builtins/kotlin/String.kt

@@ -12,8 +12,11 @@ import kotlin.math.min
  * The `String` class represents character strings. All string literals in Kotlin programs, such as `"abc"`, are
  * implemented as instances of this class.
  */
-public class String internal constructor(internal val chars: CharArray) : Comparable<String>, CharSequence {
-    public companion object;
+public class String private constructor(internal val chars: CharArray) : Comparable<String>, CharSequence {
+    public companion object {
+        // Note: doesn't copy the array, use with care.
+        internal fun unsafeFromCharArray(chars: CharArray) = String(chars)
+    }
 
     /**
      * Returns a string obtained by concatenating this string with the string representation of the given [other] object.
@@ -68,4 +71,4 @@ public class String internal constructor(internal val chars: CharArray) : Compar
     }
 }
 
-internal fun stringLiteral(startAddr: Int, length: Int) = String(unsafeRawMemoryToCharArray(startAddr, length))
+internal fun stringLiteral(startAddr: Int, length: Int) = String.unsafeFromCharArray(unsafeRawMemoryToCharArray(startAddr, length))