[Wasm][Stdlib] Use array copy in String.toCharArray()

And don't use copy during internal non-mutating algorithms

libraries/stdlib/wasm/internal/kotlin/wasm/internal/ImportExport.kt

@@ -14,7 +14,7 @@ internal fun exportString(src: String?): Int {
 
     val retAddr = unsafeGetScratchRawMemory(INT_SIZE_BYTES + src.length * CHAR_SIZE_BYTES)
     wasm_i32_store(retAddr, src.length)
-    unsafeCharArrayToRawMemory(src.toCharArray(), retAddr + INT_SIZE_BYTES)
+    unsafeCharArrayToRawMemory(src.chars, retAddr + INT_SIZE_BYTES)
     return retAddr
 }
 

libraries/stdlib/wasm/src/kotlin/Text.kt

@@ -288,7 +288,7 @@ public actual fun CharArray.concatToString(startIndex: Int, endIndex: Int): Stri
  */
 @SinceKotlin("1.3")
 @ExperimentalStdlibApi
-public actual fun String.toCharArray(): CharArray = this.chars
+public actual fun String.toCharArray(): CharArray = this.chars.copyOf()
 
 /**
  * Returns a [CharArray] containing characters of this string or its substring.