Merging Core: String into Core: XPCOM

[Nika Layzell]

Historically our XPCOM string types (nsString and friends) have been maintained in a separate module from the rest of XPCOM: "Core: String". The owners of this module have not been active in the project for the last few years, and maintenance of the String types has been handled by myself and the other XPCOM peers.

I propose merging the Core: String module into Core: XPCOM. Unless I hear objections, I plan to make this change by the end of the week.

-nika