Counts internally use uint64_t instead of size_t

This ensures that even for 32 bit platforms, the assertion count
should not plausibly overflow.

src/catch2/catch_totals.cpp

@@ -24,7 +24,7 @@ namespace Catch {
         return *this;
     }
 
-    std::size_t Counts::total() const {
+    std::uint64_t Counts::total() const {
         return passed + failed + failedButOk;
     }
     bool Counts::allPassed() const {