Add generic generator modifiers

This means mutiple generic generators and some inference helper
* take(n, generator)
* filter(predicate, generator)
* map(func, generator)
* repeat(generator, repeats)

include/internal/catch_generators.cpp

@@ -16,6 +16,10 @@ namespace Catch {
 
 IGeneratorTracker::~IGeneratorTracker() {}
 
+const char* GeneratorException::what() const noexcept {
+    return m_msg;
+}
+
 namespace Generators {
 
     GeneratorUntypedBase::~GeneratorUntypedBase() {}