Case for std.experimental

[Dragos Carp via Digitalmars-d]

> Now that I see several comments here seeking for certain 
> stability even in std.experimental and can understand why later 
> exposure can be a good thing. That, however, makes me even more 
> convinced that "experimental" is a terrible name for that 
> package and we are using it purely as staging are instead.

I think that if we rename std.experimental as std.stagging, we 
can consider that  it has absolved from its own 
expe\b\b\b\bstaging phase.

Just now this will cause no code breakage, and hopefully it will 
also avoid some confusions and discussions in the future.