Deprecate std.c.string and move its contents to druntime

When I tried to modify this file in another pull, I was told it shouldn't be used anymore, so I'm moving its contents to druntime.  This pull will need to be merged alongside [druntime #796](https://github.com/D-Programming-Language/druntime/pull/796) which puts these `strerror_r` declarations in `core.stdc.string`.

I'm not sure what should be done with the `memicmp` declaration, which isn't used anywhere in phobos.  If there's a chance that outside code might be using it, I can move it to druntime also.  Otherwise, maybe the `memicmp` declaration can be deleted altogether.

Also, if there's no chance of outside code importing `std.c.string`, maybe I can just remove the file without deprecating it and change all the imports in phobos to use `core.stdc.string` directly.

Let me know what's best, just trying to do some cleanup.