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.
Add expected failure annotation on "fixed" example to placate 'make check'.
librustc: De-export driver. rs=deexport
librustc: De-export back, lib, and util. rs=deexporting
librustc: De-export front, borrowck, and typeck/check. rs=deexporting
librustc: De-export typeck. rs=deexporting
librustc: De-export metadata. rs=deexporting
Merge pull request #4669 from pnkfelix/tutorial-revisions

Small fixes to code samples in Tutorials
treemap: rm a bit of redundant code
update comments documenting issue #4492 workaround
add is_subset and is_superset to the Set trait
add is_disjoint to the Set trait
Add Felix Klock to AUTHORS
add difference and symmetric_difference to Set
add intersection and union to the Set trait
De-capitalize "note" to get rid of spurious tidy warnings
rustdoc: Remove structural records from tests. Fixes build breakage
std: Stop using oldcomm