Refactoring of TypeInference (#76)

- Split large mutually-recursive block in `TypeInference.Expr` into
  several modules: `Def`, `Expr`, `MatchClause` and `PolyExpr`
- The mutual recursion is obtained by recursive first-class module of
  type `TypeCheckFix.tcfix` created in `TypeInference.Main`
- Moved extraction of type hints to separate module

src/TypeInference/Def.mli

@@ -0,0 +1,24 @@
+(* This file is part of DBL, released under MIT license.
+ * See LICENSE for details.
+ *)
+
+(** Type-inference for definitions *)
+
+open Common
+open TypeCheckFix
+
+(** Check type and effect of a single definition. It uses bidirectional
+  type checking, and pass the extended environment to the body-generating
+  continuation. *)
+val check_def : tcfix:tcfix ->
+  Env.t -> ImplicitEnv.t -> S.def ->
+    (T.typ, 'dir) request -> T.effrow -> def_cont ->
+      T.expr * (T.typ, 'dir) response * ret_effect
+
+(** Check type and effect of a block of definitions. It uses bidirectional
+  type checking, and pass the extended environment to the body-generating
+  continuation. *)
+val check_defs : tcfix:tcfix ->
+  Env.t -> ImplicitEnv.t -> S.def list ->
+    (T.typ, 'dir) request -> T.effrow -> def_cont ->
+      T.expr * (T.typ, 'dir) response * ret_effect