Remove the extension traits for BodyReader

[Ogeon]

Some of the functionality of BodyReader is currently implemented through extension traits, but there is no real reason to do this. I did once have a thought that it could be useful for modularity, but Rutful itself isn't using them in that manner, so it's better to leave it to the users to define their own traits if necessary.

This should be fairly simple, I think, but it may require some fiddling with conditional compiling. Here is a brief check list:

• Move the methods from ExtQueryBody and ExtJsonBody to BodyReader.
• Rename the previous methods to remove the _body suffix. Even better names are welcome.
• Make sure that the method descriptions makes sense, and remove any other references to the removed traits. I may have mentioned them in the context module, for example.
• Update the examples.