lib: move assertMsg and assertOneOf to their own library file

Since the `assertOneOf` uses `lib.generators`, they are not really trivial
anymore and should go into their own library file.

lib/asserts.nix

@@ -0,0 +1,44 @@
+{ lib }:
+
+rec {
+
+  /* Print a trace message if pred is false.
+     Intended to be used to augment asserts with helpful error messages.
+
+     Example:
+       assertMsg false "nope"
+       => false
+       stderr> trace: nope
+
+       assert (assertMsg ("foo" == "bar") "foo is not bar, silly"); ""
+       stderr> trace: foo is not bar, silly
+       stderr> assert failed at …
+
+     Type:
+       assertMsg :: Bool -> String -> Bool
+  */
+  # TODO(Profpatsch): add tests that check stderr
+  assertMsg = pred: msg:
+    if pred
+    then true
+    else builtins.trace msg false;
+
+  /* Specialized `assertMsg` for checking if val is one of the elements
+     of a list. Useful for checking enums.
+
+     Example:
+       let sslLibrary = "libressl"
+       in assertOneOf "sslLibrary" sslLibrary [ "openssl" "bearssl" ]
+       => false
+       stderr> trace: sslLibrary must be one of "openssl", "bearssl", but is: "libressl"
+
+     Type:
+       assertOneOf :: String -> ComparableVal -> List ComparableVal -> Bool
+  */
+  assertOneOf = name: val: xs: assertMsg
+    (lib.elem val xs)
+    "${name} must be one of ${
+      lib.generators.toPretty {} xs}, but is: ${
+        lib.generators.toPretty {} val}";
+
+}