Implements zero-copy html parsing.

Instead of using strings, html5ever now uses Iobufs and Spans over
Iobufs to represent the raw html data. This allows us to do all of
the parsing without copying the HTML into a tree of strings: it can
be Spans and Iobufs all the way down.

There were several performance hacks done to get this faster. Most
of them were to work around rustc failures as described in:

http://discuss.rust-lang.org/t/the-sad-state-of-zero-on-drop/944

Here's a little list of transformations done for performance reasons:

  - State machine states are now functions. Rustc isn't smart enough
    to properly handle large-ish things on the stack in different
    match arms. This does mean that it's sane to inline the jump
    table into `feed`, which had a nice impact on performance. Jump
    tables inside loops are especially efficient because it's just
    a bigger jump table!
  - Things which get atomized anyhow (except for doctypes, which
    weren't hot enough for me to bother changing) use the old String
    parsing method, since it ends up being a lot faster for small
    strings and doesn't cause O(tags) allocations, thanks to truncation.
  - A custom Option called `FastOption` which doesn't zero on `take`
    and can't be matched on, but still maintains safety.
  - Iobufs in the input_buffers Ringbuf are padded to 32 bytes, to
    allow indexing without a multiply. That was actually a hotspot
    that showed up in perf, which is a little scary.
  - No utf-8 decoding of the chars is avoided unless absolutely
    necessary. For most parsing, we just need the utf-8 length,
    which is much easier to calculate (a branch, and a LUT on the
    first byte in the "slow" path).
  - Chars and Runs are parsed into a "shared" location every time,
    because rustc is really bad at generating code for types which
    Drop a lot in a loop. See the discuss post at the top.
  - A new temp_buf has been introduced, because it is no longer
    performant to just append random characters to spans. Consider
    a partially-consumed comment start: `<-`. If the next character
    is an `a`, the <- needs to be emitted. The second temporary
    buffer is used to handle cases like that.
  - Similar to above, but when parsing char refs: the '&' and '#'
    are saved in case of backout.
  - Dashes at the end of a comment `----->` need to be saved and
    shuffled as we keep reading more dashes, so that we always
    emit the "right" ones to keep the span continuous. This
    required a little 2-element "queue": `first_comment_end_dash`
    and `second_comment_end_dash`.
  - Some of the tokenizer fields were reordered for cache
    efficiency.
  - Some inliner guidance was done in `get_char` and
    `get_preprocessed_char`, to keep fast paths fast.
  - `clone_from` to get data out of the input buffers is used
    where it makes sense, preventing a bunch of bad rustc codegen.

As a result of these optimizations (and zero-copy parsing in general):

===

zero-copy

test tokenize uncommitted/html5.html       ... bench: 124076195 ns/iter (+/- 9519897)
test tokenize uncommitted/lipsum-1M.html   ... bench:   1989708 ns/iter (+/- 405327)
test tokenize uncommitted/sina.com.cn.html ... bench:   7210262 ns/iter (+/- 1391972)
test tokenize uncommitted/strong.html      ... bench:  30002001 ns/iter (+/- 3375152)
test tokenize uncommitted/webapps.html     ... bench:  99264377 ns/iter (+/- 8138989)
test tokenize uncommitted/wikipedia.html   ... bench:   3841740 ns/iter (+/- 612645)

original

test tokenize uncommitted/html5.html       ... bench: 153991836 ns/iter (+/- 7196531)
test tokenize uncommitted/lipsum-1M.html   ... bench:   2393385 ns/iter (+/- 450953)
test tokenize uncommitted/sina.com.cn.html ... bench:   8837605 ns/iter (+/- 1238217)
test tokenize uncommitted/strong.html      ... bench:  44153393 ns/iter (+/- 5076161)
test tokenize uncommitted/webapps.html     ... bench: 136860951 ns/iter (+/- 8137049)
test tokenize uncommitted/wikipedia.html   ... bench:   4868854 ns/iter (+/- 797178)

SUMMARY

html5.html:       19%
sina.com.cn.html: 14%
strong.html:      47%
webapps.html:     27%
wikipedia.html:   21%
lipsum-1M.html:   17%

====

r? @kmcallister