move std.{ascii, base64, utf} to a new encoding package

[Seb via Digitalmars-d]

Hi all,

today Johannes and I had a short discussion on Github about 
std.base64. I think his post is a good summary of the status quo:

> base64 shouldn't be a top level module though, moving it to a 
> new package std.encoding may be a good idea. std.base64 is just 
> quite old and when it was written we didn't use subpackages in 
> phobos yet. std.utf and std.ascii are also encodings and could 
> be moved to std.encoding as well, AFAICS. And we also have a 
> std.encoding module which should be part of the same package.

That being said I know that such housekeeping isn't liked a lot, 
but with `public import` we can very easily mitigate breakage and 
allow a long deprecation period.
What's your opinion on this? Any major objections?

Cheers,

Seb

[1] 
https://github.com/dlang/phobos/pull/4720#issuecomment-238348553