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_list.h

@@ -23,10 +23,10 @@ namespace Catch {
     };
 
     struct TagInfo {
-        void add(std::string const& spelling);
+        void add(StringRef spelling);
         std::string all() const;
 
-        std::set<std::string> spellings;
+        std::set<StringRef> spellings;
         std::size_t count = 0;
     };