Ensure that the docs examples verify as Cretonne IL.

Any *.cton files in the docs directory are now included when running the
test-all.sh script. This is to ensure that the examples are in fact
correct IL.

Always print NaN and Inf floats with a sign. Print the positive ones as
+NaN and +Inf to make them easier to parse.

docs/langref.rst

@@ -31,8 +31,7 @@ Here is the same function compiled into Cretonne IL:
 
 .. literalinclude:: example.cton
     :language: cton
-    :linenos:
-    :emphasize-lines: 2
+    :lines: 2-
 
 The first line of a function definition provides the function *name* and
 the :term:`function signature` which declares the argument and return types.