Fix two code examples in main rust tutorial.

1. The section on trait definitions of static methods should include
   a trait with a static method in the generated document.

2. The section on trait inheritance had a expression that appears
   nonsensical ("let mycircle = @mycircle") in the generated document.
   The text would be clearer (IMO) if we continued with the running
   example of CircleStruct.
Fixed two examples of erroneous code so their errors match expectation.

1. In the first case, the previous code was failing during type inference
   due to mismatched structure.  Fix is to use the X structure at both
   points in the code.

2. In the second case, a naive transcription that subsitutes *nothing*
   in for the omitted statements signified by "..." will actually
   compile without an error.  Furthermore, any pure code could also be
   substituted for the ellipsis and the code would compile (as the
   text already states).  So to make the example more illustrative, it
   would be better to include an impure callback, which makes the
   potential for aliasing immediately obvious to the reader.
Formatting cleanup.

I had put a line break in to try to stress the binding of mycircle,
but generated document looks cleaner without newline.