Make StringRef's operator std::string explicit

This way it is explicit when there is a `StringRef` -> `std::string`
conversion and makes it easier to look for allocations that could
be avoided.

Doing this has already removed one allocation per registered test
case, as there was a completely pointless `StringRef` -> `std::string`
conversion when parsing tags of a test case.

include/internal/catch_stringref.h

@@ -23,6 +23,7 @@ namespace Catch {
     class StringRef {
     public:
         using size_type = std::size_t;
+        using const_iterator = const char*;
 
     private:
         friend struct StringRefTestAccess;
@@ -78,7 +79,7 @@ namespace Catch {
             return *this;
         }
 
-        operator std::string() const;
+        explicit operator std::string() const;
 
         void swap( StringRef& other ) noexcept;
 
@@ -105,6 +106,10 @@ namespace Catch {
         // Note that the pointer can change when if the StringRef is a substring
         auto currentData() const noexcept -> char const*;
 
+    public: // iterators
+        const_iterator begin() const { return m_start; }
+        const_iterator end() const { return m_start + m_size; }
+
     private: // ownership queries - may not be consistent between calls
         auto isOwned() const noexcept -> bool;
         auto isSubstring() const noexcept -> bool;