WASM: Implement wasm-native strings part 1

There are several changes here but they all required for at least one test to pass.
- Implemented String class and several utility functions using built-in CharArray
- Added new constant memory segment to hold string literals and required funcs to work with them
- Added very crude mostly incorrect rudimentary ability to pass strings back to javascript

js/js.tests/test/org/jetbrains/kotlin/js/test/BasicWasmBoxTest.kt

@@ -151,7 +151,7 @@ abstract class BasicWasmBoxTest(
             const wasmModule = new WebAssembly.Module(wasmBinary);
             const wasmInstance = new WebAssembly.Instance(wasmModule, { runtime, js_code });
 
-            const actualResult = wasmInstance.exports.$testFunction();
+            const actualResult = runtime.importStringToJs(wasmInstance.exports.$testFunction(), wasmInstance.exports.memory);
             if (actualResult !== "OK")
                 throw `Wrong box result '${'$'}{actualResult}'; Expected "OK"`;
         """.trimIndent()