This patch makes DOMString an opaque wrapper round String (currently it's a transparent wrapper).
The changes are:
* Replacing DOMString(foo) by DOMString::from(foo).
* Replacing foo.0 by String::from(foo).
* Adding functions clear, push_str and extend for in-place mutation of DOMStrings.
* Replacing DOMString by String in other threads (devtools, storage and filereader).
* Making DOMString implement !Send.
* Removing the pub attribute from the contents of DOMString.
This enables experimenting with other string representations in the DOM.
Source-Repo: https://github.com/servo/servo
Source-Revision: 62acdd303b78951885c2c90747b31f318907d6c9
