Gather data on string interning in Gecko

[kmcallister]

For designing our string interning system (#1153) it would be good to have data on which strings Gecko interns, for example

• How often are they statically known strings (e.g. well-known tags and attributes)?
• What is the distribution of lengths?

I don't know if this is really E-easy, but it might be a good first project for a current Gecko contributor who wants to help out with Servo.

See also the mailing list discussion.

[glennw]

Should we close this, or do you think it would still be useful?

[kmcallister]

string-cache itself can now provide the info we need. See servo/string-cache#37.