Integrate front::test and rustpkg into the syntax extension framework

[brson]

Both --test and now rustpkg apply a type of syntax extension that is different from those in ext. These apply some global transformations, including replacing main, so they aren't applied by default. The test extension is actually a compiler pass that runs on all input, but it only does anything when in the test configuration--cfg test (indirectly by --test). rustpkg is similar but driven by the rustpkg command. I imagine there will be others.

[catamorphism]

Added far-future milestone; it doesn't seem like we'd be embarrassed to call Rust production-ready without this.

[alexcrichton]

I was thinking about this the other day, and I believe that it should be possible for completely removing all test-related infrastructure from rustc. We would be distributing another tool, rusttest which is probably too painful to use, but there's no real reason for the compiler itself to know anything about a test harness (this is all very specific to the extra::test implementation and prevents other implementations of a test harness).

[erickt]

Visiting for triage. rustpkg is gone, but rustc::front::test is still around and could be factored out.

[steveklabnik]

Re-traige: it seems rustc::front is gone too, so I'm iving this a close.