[stdlib] Add CT @in Macro/Builtin (#2662)

* [stdlib] Add CT `@in` Macro/Builtin

lib/std/core/builtin.c3

@@ -93,6 +93,21 @@ macro usz bitsizeof($Type) @builtin @const => $Type.sizeof * 8u;
 
 macro usz @bitsizeof(#expr) @builtin @const => $sizeof(#expr) * 8u;
 
+<*
+ Compile-time check for whether a set of constants contains a certain expression.
+
+ @param #needle : "The expression whose value should be located."
+*>
+macro bool @in(#needle, ...) @builtin @const
+{
+    $for var $x = 0; $x < $vacount; $x++:
+        $assert $defined(#needle == $vaconst[$x])
+            : "Index %s: types '%s' (needle) and '%s' are not equatable", $x, $typeof(#needle), $typeof($vaconst[$x]);
+        $if #needle == $vaconst[$x]: return true; $endif
+    $endfor
+    return false;
+}
+
 <*
  Convert an `any` type to a type, returning an failure if there is a type mismatch.