Implement a web backend for rustdoc_ng

This large commit implements and `html` output option for rustdoc_ng. The
executable has been altered to be invoked as "rustdoc_ng html <crate>" and
it will dump everything into the local "doc" directory. JSON can still be
generated by changing 'html' to 'json'.

This also fixes a number of bugs in rustdoc_ng relating to comment stripping,
along with some other various issues that I found along the way.

The `make doc` command has been altered to generate the new documentation into
the `doc/ng/$(CRATE)` directories.

src/libstd/macros.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #[macro_escape];
+#[doc(hidden)];
 
 macro_rules! rterrln (
     ($( $arg:expr),+) => ( {