[boost] Removal of boost::basic_string_ref (in favor of basic_string_view)

Subject: [boost] Removal of boost::basic_string_ref (in favor of basic_string_view)
From: Marshall Clow (mclow.lists_at_[hidden])
Date: 2016-02-17 14:52:42

I have just removed boost::basic_string_ref from the develop branch, and
updated the specializations to use basic_string_view.

Code that uses boost::string_ref (and wstring_ref) should continue to work
as before.

The basic_string_view class better matches what the standards committee is
doing and has WAY more constexpr support.

I'll be watching the testbots for results, but I believe that the only part
of boost that was using basic_string_ref was Boost.Log, and that was
switched over to basic_string_view in December.

-- Marshall