Add 'process' module with 'exit' function (#541)

libraries/common/process.effekt

@@ -0,0 +1,10 @@
+module process
+
+extern io def exit(errorCode: Int): Nothing =
+  js   "(function() { process.exit(${errorCode}) })()"
+  llvm """
+    call void @exit(i64 %errorCode)
+    ret %Pos zeroinitializer
+  """
+  chez "(exit ${errorCode})"
+  ml "(Posix.Process.exit (Word8.fromInt ${errorCode}))"

libraries/common/test.effekt

@@ -1,4 +1,5 @@
 import string
+import process
 
 interface Assertion {
   def assert(condition: Bool, msg: String): Unit
@@ -87,4 +88,11 @@ def suite(name: String) { body: => Unit / Test }: Bool = {
     println(ANSI::GREEN ++ "All tests passed (" ++ passed.show ++ " passed)" ++ ANSI::RESET)
     true
   }
-}
+}
+
+/// Use as `def main() = mainSuite("...") { ... }`
+def mainSuite(name: String) { body: => Unit / Test }: Unit = {
+  val result = suite(name) { body() }
+  val exitCode = if (result) 0 else 1
+  exit(exitCode)
+}