Optimize CPointer.toKStringFromUtf8 on Native

Use strlen and stop allocating intermediate ByteArray
to make the performance comparable to ByteArray.decodeToString().

Partially fix KT-44357.

kotlin-native/runtime/src/main/cpp/Interop.cpp

@@ -17,8 +17,10 @@
 #include <assert.h>
 #include <stdio.h>
 #include <stdint.h>
+#include <string.h>
 
 #include "Alloc.h"
+#include "KString.h"
 #include "Memory.h"
 #include "MemorySharedRefs.hpp"
 #include "Types.h"
@@ -42,4 +44,8 @@ OBJ_GETTER(Kotlin_Interop_derefStablePointer, KNativePtr pointer) {
   RETURN_OBJ(holder->ref<ErrorPolicy::kThrow>());
 }
 
+OBJ_GETTER(Kotlin_CString_toKStringFromUtf8Impl, const char* cstring) {
+  RETURN_RESULT_OF(StringFromUtf8Buffer, cstring, strlen(cstring));
+}
+
 }

kotlin-native/runtime/src/main/cpp/KString.cpp

@@ -511,6 +511,13 @@ OBJ_GETTER(Kotlin_ByteArray_unsafeStringFromUtf8, KConstRef thiz, KInt start, KI
   RETURN_RESULT_OF(utf8ToUtf16, rawString, size);
 }
 
+OBJ_GETTER(StringFromUtf8Buffer, const char* start, size_t size) {
+  if (size == 0) {
+    RETURN_RESULT_OF0(TheEmptyString);
+  }
+  RETURN_RESULT_OF(utf8ToUtf16, start, size);
+}
+
 OBJ_GETTER(Kotlin_String_unsafeStringToUtf8, KString thiz, KInt start, KInt size) {
   RETURN_RESULT_OF(unsafeUtf16ToUtf8Impl<utf8::with_replacement::utf16to8>, thiz, start, size);
 }

kotlin-native/runtime/src/main/cpp/KString.h

@@ -31,6 +31,8 @@ OBJ_GETTER(CreateStringFromUtf8, const char* utf8, uint32_t lengthBytes);
 char* CreateCStringFromString(KConstRef kstring);
 void DisposeCString(char* cstring);
 
+OBJ_GETTER(StringFromUtf8Buffer, const char* start, size_t size);
+
 #ifdef __cplusplus
 }
 #endif