hdl.ast: allow slicing [n:n] into n-bit value.

nmigen/hdl/ast.py

@@ -361,7 +361,7 @@ class Slice(Value):
             raise TypeError("Slice end must be an integer, not '{!r}'".format(end))
 
         n = len(value)
-        if start not in range(-n, n):
+        if start not in range(-(n+1), n+1):
             raise IndexError("Cannot start slice {} bits into {}-bit value".format(start, n))
         if start < 0:
             start += n