More Test::Stream::Toolset usage improvements.

Rather than hackily using Test::Stream::Toolset as a base class and
using import::into to load it into the target, Test::Kit now correctly
uses Test::Stream::Toolset export_into() and Test::Stream::Exporter
export_into() to get the behaviour we want into the target kit.
Update POD section on Test::Builder 1.3 compatibility

Remove all mentions of Test::Builder::Provider from the documentation
and replace with references to Test::Stream::Toolset/Exporter.