wip: Split fuzz.rs into individual fuzz targets with cargo-fuzz

[mozfreddyb]

I've ommitted the test for tendril.clear() as I fail to understand the risk associated with its implementation.

I'm also ommitting this block, as I'm failing to understand its utility:

            32...47 => {
                let lenstr = format!("[length = {}]", buf_tendril.len());
                buf_string.push_str(&lenstr);
                buf_tendril.push_slice(&lenstr);
                assert_eq!(&*buf_string, &*buf_tendril);
            }

The "[length = ..]" thing does not seem to cause any different behavior. There is no syntax within tendrils that I could find.

NB: As a follow-up I would like to format the whole crate with rustfmt, as it would make it tremendously easier to develop code while rustfmt is running on the whole repo.

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This change is 

[Manishearth]

Note: this repo has been moved to https://github.com/servo/html5ever