Test: promote check to library

(cherry picked from commit a74e7ce)

Author: kavon

test/Inputs/Swiftskell.swift

@@ -55,6 +55,17 @@ public protocol Generator: ~Copyable {
   func next() -> Maybe<Element>
 }
 
+/// Eager assertion function, to avoid autoclosures.
+public func check(_ result: Bool, _ string: String? = nil,
+                  _ file: String = #file, _ line: Int = #line) {
+  if result { return }
+  var msg = "assertion failure (\(file):\(line))"
+  if let extra = string {
+    msg += ":\t" + extra
+  }
+  fatalError(msg)
+}
+
 // MARK: Tuples
 public enum Pair<L: ~Copyable, R: ~Copyable>: ~Copyable {
   case pair(L, R)

test/Interpreter/moveonly_swiftskell.swift

@@ -20,16 +20,6 @@
 
 import Swiftskell
 
-/// assertion function
-func check(_ result: Bool, _ string: String? = nil, _ line: Int = #line) {
-  if result { return }
-  var msg = "assertion failure (line \(line))"
-  if let extra = string {
-    msg += ":\t" + extra
-  }
-  fatalError(msg)
-}
-
 /// Basic noncopyable type for testing.
 struct File: ~Copyable, Show {
   let id: Int