restore type inference layer 0

The type IR, unification with the unknown type, constraints and
schemes, built and tested on its own. Nothing calls it yet.

utils.scm

@@ -138,6 +138,17 @@ wrap a form-building expression."
   "Signal an error about FORM, prefixed with where it was written."
   (apply error (string-append (form-location form) message) args))
 
+(define (sex-warning form message . args)
+  "Report something about FORM that does not stop the compilation.
+Goes to stderr, prefixed with where the form was written, so a warning
+reads like an error and sorts alongside one in a build log."
+  (let ((port (current-error-port)))
+    (display (form-location form) port)
+    (display "warning: " port)
+    (display message port)
+    (for-each (lambda (arg) (display " " port) (display arg port)) args)
+    (newline port)))
+
 (define (stamp-form-source! form src)
   "Give FORM and every subform that has none the location SRC. Used for
 macro expansions, which inherit the location of the call site the way a