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.

tests/infer.module.scm

@@ -0,0 +1,44 @@
+(module infer
+    (;; The IR
+     tvar?
+     tvar-id
+     tvar-classes
+     tvar-rigid?
+     fresh-tvar
+     fresh-rigid-tvar
+
+     prim-type? prim-name prim-quals make-prim
+     ptr-type? ptr-target ptr-quals make-ptr
+     array-type? array-elt array-size make-array-type
+     fn-type? fn-ret fn-args fn-variadic? make-fn-type
+     agg-type? agg-kind agg-name agg-spelling agg-quals make-agg
+     alias-type? alias-name alias-expansion alias-quals make-alias
+     unknown-type? the-unknown-type
+
+     resolve
+     underlying
+     type-quals
+     free-tvars
+     decay
+
+     ;; The boundary
+     parse-type
+     unparse-type
+
+     ;; Constraints
+     register-class!
+     add-instance!
+     entails?
+     default-tvar!
+     default-type-variables!
+
+     ;; Unification
+     unify
+
+     ;; Type schemes
+     scheme? scheme-vars scheme-constraints scheme-type
+     make-scheme
+     generalize
+     instantiate
+     substitute)
+  "../infer.scm")