refactor(library/data/unit): break unit.lean into smaller pieces

The unit datatype is used by automation.

We want to be able to access its declaration without having to access
all dependencies (e.g., decidable, subsingleton, inhabited, ...).

This is *not* an optimization, but a way to make sure we can "import" unit
before we import other declarations.

library/data/unit/basic.lean

@@ -0,0 +1,5 @@
+-- Copyright (c) 2014 Microsoft Corporation. All rights reserved.
+-- Released under Apache 2.0 license as described in the file LICENSE.
+-- Author: Leonardo de Moura
+inductive unit.{l} : Type.{l} :=
+  star : unit

library/data/unit/decl.lean

@@ -0,0 +1,9 @@
+-- Copyright (c) 2014 Microsoft Corporation. All rights reserved.
+-- Released under Apache 2.0 license as described in the file LICENSE.
+-- Author: Leonardo de Moura
+inductive unit.{l} : Type.{l} :=
+  star : unit
+
+namespace unit
+  notation `⋆` := star
+end unit

library/data/unit/default.lean

@@ -0,0 +1,4 @@
+-- Copyright (c) 2014 Microsoft Corporation. All rights reserved.
+-- Released under Apache 2.0 license as described in the file LICENSE.
+-- Author: Leonardo de Moura
+import data.unit.decl data.unit.thms data.unit.insts

library/data/unit/insts.lean

@@ -1,24 +1,13 @@
 -- Copyright (c) 2014 Microsoft Corporation. All rights reserved.
 -- Released under Apache 2.0 license as described in the file LICENSE.
 -- Author: Leonardo de Moura
-import logic.decidable logic.inhabited
+import data.unit.decl data.unit.thms logic.decidable logic.inhabited
 open decidable
 
-inductive unit.{l} : Type.{l} :=
-  star : unit
 namespace unit
-  notation `⋆` := star
-
-  -- remove duplication?
-  protected theorem equal (a b : unit) : a = b :=
-  rec_on a (rec_on b rfl)
-
   protected theorem subsingleton [instance] : subsingleton unit :=
   subsingleton.intro (λ a b, equal a b)
 
-  theorem eq_star (a : unit) : a = star :=
-  equal a star
-
   protected definition is_inhabited [instance] : inhabited unit :=
   inhabited.mk ⋆