[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/number/assignmentIntOverflow.kt

@@ -0,0 +1,26 @@
+// EXPECTED_REACHABLE_NODES: 1282
+package foo
+
+fun bigValue() = 0x7FFFFFFC
+
+fun mediumValue() = 0x12345
+
+fun box(): String {
+    var v = bigValue()
+    v += 1
+    if (v != 0x7FFFFFFD) return "fail1"
+
+    v = bigValue()
+    v += 8
+    if (v != -0x7FFFFFFC) return "fail2"
+
+    v = mediumValue()
+    v *= 0x23456
+    if (v != -2112496338) return "fail3"
+
+    v = bigValue()
+    v *= bigValue()
+    if (v != 16) return "fail4"
+
+    return "OK"
+}