[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

js/js.translator/testData/wasmBox/passedCommonTests/ir/closureConversion/closureConversion1.kt

@@ -0,0 +1,6 @@
+fun foo(x: String): String {
+    fun bar(y: String) = x + y
+    return bar("K")
+}
+
+fun box() = foo("O")

js/js.translator/testData/wasmBox/passedCommonTests/ir/closureConversion/closureConversion3.kt

@@ -0,0 +1,11 @@
+fun foo(x: String): String {
+    fun bar(y: String): String {
+        fun qux(z: String): String =
+                x + y + z
+        return qux("")
+    }
+    return bar("K")
+}
+
+fun box(): String =
+        foo("O")

js/js.translator/testData/wasmBox/passedCommonTests/ir/closureConversion/closureConversion4.kt

@@ -0,0 +1,6 @@
+fun String.foo(): String {
+    fun bar(y: String) = this + y
+    return bar("K")
+}
+
+fun box() = "O".foo()