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/__init__.py

@@ -12,6 +12,11 @@ instructions.
 class Type(object):
     """A concrete value type."""
 
+    def __init__(self, name, membytes, doc):
+        self.name = name
+        self.membytes = membytes
+        self.__doc__ = doc
+
     def __str__(self):
         return self.name
 
@@ -19,12 +24,12 @@ class ScalarType(Type):
     """
     A concrete scalar (not vector) type.
 
-    Also tracks a unique set of :class:`VectorType` instances with this type as
-    the lane type.
+    Also tracks a unique set of :py:class:`VectorType` instances with this type
+    as the lane type.
     """
 
-    def __init__(self, name):
-        self.name = name
+    def __init__(self, name, membytes, doc):
+        super(ScalarType, self).__init__(name, membytes, doc)
         self._vectors = dict()
 
     def __repr__(self):
@@ -53,9 +58,14 @@ class VectorType(Type):
 
     def __init__(self, base, lanes):
         assert isinstance(base, ScalarType), 'SIMD lanes must be scalar types'
+        super(VectorType, self).__init__(
+                name='{}x{}'.format(base.name, lanes),
+                membytes=lanes*base.membytes,
+                doc="""
+                A SIMD vector with {} lanes containing a {} each.
+                """.format(lanes, base.name))
         self.base = base
         self.lanes = lanes
-        self.name = '{}x{}'.format(base.name, lanes)
 
     def __repr__(self):
         return 'VectorType(base={}, lanes={})'.format(self.base.name, self.lanes)
@@ -65,7 +75,10 @@ class IntType(ScalarType):
 
     def __init__(self, bits):
         assert bits > 0, 'IntType must have positive number of bits'
-        super(IntType, self).__init__('i{:d}'.format(bits))
+        super(IntType, self).__init__(
+                name='i{:d}'.format(bits),
+                membytes=bits/8,
+                doc="An integer type with {} bits.".format(bits))
         self.bits = bits
 
     def __repr__(self):
@@ -74,9 +87,9 @@ class IntType(ScalarType):
 class FloatType(ScalarType):
     """A concrete scalar floating point type."""
 
-    def __init__(self, bits):
+    def __init__(self, bits, doc):
         assert bits > 0, 'FloatType must have positive number of bits'
-        super(FloatType, self).__init__('f{:d}'.format(bits))
+        super(FloatType, self).__init__( name='f{:d}'.format(bits), membytes=bits/8, doc=doc)
         self.bits = bits
 
     def __repr__(self):