Implement atoms for parsing HTML, comparing Attrs, etc

[burg]

In places like the HTML parser (hubbub_html_parser.rs), many redundant ~strs are created simply for the purpose of matching tag names and attributes. This should instead be done with atoms.

To implement atoms, we may need to port something like gperf (http://www.gnu.org/software/gperf/) to Rust, or have some other solution for fast string lookup.

[ILyoan]

How about using trie like data structure?

[SimonSapin]

Is this the same as #282?

[tetsuharuohzeki]

From irc logs:

2 minutes ago tetsuharu:What is this "atom" meaning?: #85
2 minutes ago SimonSapin:tetsuharu: interned string

[kmcallister]

Folding this into #1153.

port something like gperf (http://www.gnu.org/software/gperf/) to Rust

I'm using rust-phf for various static string tables in the new HTML5 parser.