Improvements to std.string

[Jonathan M Davis]

https://github.com/D-Programming-Language/phobos/pull/101

I made several improvements to std.string, std.uni, and std.ctype - primarily 
with the aim of fixing function names to be properly camelcased and improving 
unicode support - and while the changes are generally fairly simple and 
generally well-organized, there are quite a few of them, and I think that the 
changes could use some extra eyes. Also, there are probably a few changes 
which deserve some discussion.

For instance, a couple of items which already came up

1. Would it be better to rename toStringz to toCString when fixing it so that 
it's properly camelcased (in my changes I just did toStringZ since it was the 
straightforward fix to the naem, but it's not exactly a great name to begin 
with).

2. Should std.ctype be renamed (or at least the updated functions be put into 
another module which will replace it) - e.g. std.ascii?

I don't want want a bikeshedding discussion, but it is a fairly large commit, 
and it could use a looking over by more than just a couple of Phobos devs, and 
some of the changes likely do merit some discussion. So, feel free to look 
over the pull request and make appropriate suggestions.

- Jonathan M Davis