[WASM] Initial infrastructure

- New module ":compiler:backend.wasm"
    - Initial compiler infra (driver, phaser, context)
    - Subset of Wasm AST
    - Skeleton of IR -> Wasm AST
    - Wasm AST -> WAT transformer

- Testing infra

- SpiderMonkey jsshell tool

libraries/stdlib/wasm/runtime/runtime.js

@@ -0,0 +1,45 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+const runtime = {
+    /**
+     * kotlin.String#plus(other: Any?): String
+     * @return {string}
+     */
+    String_plus(str1, str2) {
+        if (typeof str1 != "string") throw `Illegal argument str1: ${str1}`;
+        return str1 + String(str2);
+    },
+
+    /**
+     * @return {number}
+     */
+    String_getLength(str) {
+        if (typeof str != "string") throw `Illegal argument x: ${str}`;
+        return str.length;
+    },
+
+    /**
+     * @return {number}
+     */
+    String_getChar(str, index) {
+        if (typeof str != "string") throw `Illegal argument str: ${str}`;
+        if (typeof index != "number") throw `Illegal argument index: ${index}`;
+        return str.charCodeAt(index);
+    },
+
+    /**
+     * @return {number}
+     */
+    String_compareTo(str1, str2) {
+        if (str1 > str2) return 1;
+        if (str1 < str2) return -1;
+        return 0;
+    },
+
+    String_getLiteral(index) {
+        return runtime.stringLiterals[index];
+    }
+};