feat(library/standard): add bit, char, and string types

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

library/standard/bit.lean

@@ -0,0 +1,10 @@
+-- Copyright (c) 2014 Microsoft Corporation. All rights reserved.
+-- Released under Apache 2.0 license as described in the file LICENSE.
+-- Author: Leonardo de Moura
+namespace bit
+inductive bit : Type :=
+| b0 : bit
+| b1 : bit
+notation `'0` := b0
+notation `'1` := b1
+end

library/standard/standard.lean

@@ -1,2 +1,2 @@
-import logic tactic num
+import logic tactic num string
 

library/standard/string.lean

@@ -0,0 +1,14 @@
+-- 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 bit
+using bit
+
+namespace string
+inductive char : Type :=
+| ascii : bit → bit → bit → bit → bit → bit → bit → bit → char
+
+inductive string : Type :=
+| empty : string
+| str   : char → string → string
+end