Store tags in one big pre-allocated string and only work with refs

This should decrease the number of allocations before main is entered
significantly, but complicates the code somewhat in return.

Assuming I used `massif` right, doing just `SelfTest --list-tests`
went from 929 allocations at "Remove gcc-4.9 from the travis builds"
(2 commits up), to 614 allocations with this commit.

include/internal/catch_stringref.h

@@ -58,6 +58,8 @@ namespace Catch {
             return m_start[index];
         }
 
+        bool operator<(StringRef const& rhs) const noexcept;
+
     public: // named queries
         constexpr auto empty() const noexcept -> bool {
             return m_size == 0;
@@ -91,7 +93,6 @@ namespace Catch {
     auto operator += ( std::string& lhs, StringRef const& sr ) -> std::string&;
     auto operator << ( std::ostream& os, StringRef const& sr ) -> std::ostream&;
 
-
     constexpr auto operator "" _sr( char const* rawChars, std::size_t size ) noexcept -> StringRef {
         return StringRef( rawChars, size );
     }