performance improvement - StringRef::operator=

inlined and reduced data copy in half.
Further work on #1086.
Brings test from 0m44.942s to 0m37.913.

include/internal/catch_stringref.cpp

@@ -17,10 +17,6 @@
 
 namespace Catch {
 
-    auto StringRef::operator = ( StringRef other ) noexcept -> StringRef& {
-        swap( other );
-        return *this;
-    }
     StringRef::operator std::string() const {
         return std::string( m_start, m_size );
     }