librustc: Port the pretty printer annotation infrastructure to use traits instead of garbage collected functions.

librustc: Remove garbage collected functions from front/{config,test} and metadata/{tydecode,tyencode}

this is still broken on the bsd builder, perhaps because it has 1 core

Removes old rustdoc, moves rustdoc_ng into its place instead (plus drops the _ng
suffix). Also shreds all reference to rustdoc_ng from the Makefile rules.

This has to leave rust_gen_seed and rng_gen_seed around since they're
used to initialise the std::rand RNGs.

Also, documentation & general clean-up:
- remove `gen_char_from`: better served by `sample` or `choose`.
- `gen_bytes` generalised to `gen_vec`.
- `gen_int_range`/`gen_uint_range` merged into `gen_integer_range` and
  made to be properly uniformly distributed. Fixes #8644.

Minor adjustments to other functions.

Closes #9333.

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.

Closes #9379

This was a dead end experiment, and not a sensible way of implementing
generic data parallelism. This also removes the `graph500-bfs.rs`
benchmark because it relies on `extra::par`.

Closes #5626

This was, somehow, missed by #8332.

Many people will be very confused that their debug! statements aren't working
when they first use rust only to learn that they should have been building with
`--cfg debug` the entire time. This inverts the meaning of the flag to instead
of enabling debug statements, now it disables debug statements.

This way the default behavior is a bit more reasonable, and requires less
end-user configuration. Furthermore, this turns on debug by default when
building the rustc compiler.

This guarantees that if there is work to do it will be found