Merge into a single crate. Use macros even on unstable.

Breaking changes:

* `ns!("")` should be written `ns!()`
* Other `ns!(…)` macros should be lowercase, unquoted.

Fixes #124
Closes #123

r? @asajeffrey

<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/string-cache/125)
<!-- Reviewable:end -->