Removed noexcept from string_view::compare that may throw.

One overload of string_view::compare calls substr internally,
which may throw. This makes compare potentially throwing.

Fixes https://github.com/boostorg/utility/issues/94.

include/boost/utility/string_view.hpp

@@ -201,7 +201,7 @@ namespace boost {
             }
 
         BOOST_CXX14_CONSTEXPR int compare(size_type pos1, size_type n1, basic_string_view x)
-          const BOOST_NOEXCEPT {
+          const {
             return substr(pos1, n1).compare(x);
             }