Add a TypeDocumenter for Cretonne types.

Use the autodoc Sphinx module to add a .. autoctontype:: directive which
generates documentation for one of the types in the cretonne.types module.

meta/cretonne/types.py

@@ -2,19 +2,32 @@
 
 from . import ScalarType, IntType, FloatType
 
-#: A boolean value.
-bool = ScalarType('bool')
+bool = ScalarType('bool', 0,
+        """
+        A boolean value that is either true or false.
+        """)
 
-i8  = IntType(8)  #: 8-bit int.
-i16 = IntType(16) #: 16-bit int.
-i32 = IntType(32) #: 32-bit int.
-i64 = IntType(64) #: 64-bit int.
+i8  = IntType(8)
+i16 = IntType(16)
+i32 = IntType(32)
+i64 = IntType(64)
 
-f32 = FloatType(32) #: IEEE 32-bit float.
-f64 = FloatType(64) #: IEEE 64-bit float
+f32 = FloatType(32,
+        """
+        A 32-bit floating point type represented in the IEEE 754-2008 *binary32*
+        interchange format. This corresponds to the :c:type:`float` type in most
+        C implementations.
+        """)
 
-i8x16 = i8.by(16) #: Vector of 16 i8 lanes.
+f64 = FloatType(64,
+        """
+        A 64-bit floating point type represented in the IEEE 754-2008 *binary64*
+        interchange format. This corresponds to the :c:type:`double` type in
+        most C implementations.
+        """)
 
-f32x4 = f32.by(4) #: Vector of 4 f32 lanes.
-f64x2 = f64.by(2) #: Vector of 2 f64 lanes.
+i8x16 = i8.by(16)
+
+f32x4 = f32.by(4)
+f64x2 = f64.by(2)