CHECK_THAT fails to stringify a range with std::views::filter

[petr-polezhaev]

std::ranges::filter_view has only non-const begin/end as it writes the filtered value into the view object (per standard requirements), yet the Catch::stringify and Catch::StringMaker...::convert both accept const-references only and IsStreamInsertable tests for std::declval<U> only (rvalue) with const T& as argument

To fix this I had to:

1. Change stringify to universal reference and use forward
2. Move IsStreamInsertable constness to the template argument (specified by callers)
3. Use std::declval<U&> in IsStreamInsertable is is_range_impl
4. Add non-const overloads to StringMaker (normal and range specialization)
5. Add a non-const overload to ReusableStringStream::operator<<
6. Remove const from rangeToString

Test case:

#include <catch2/catch_test_macros.hpp>
#include <catch2/matchers/catch_matchers_range_equals.hpp>

TEST_CASE("std::views::filter")
{
	int const ints[] = {1, 2, 3};
	auto range = ints | std::views::filter([](int const x) { return x % 2 == 0; });
	int const expected[] = {3}; // forcing the error output
	CHECK_THAT(range, Catch::Matchers::RangeEquals(expected));
}

https://gcc.godbolt.org/z/4EexKf1d5

Expected:

CHECK_THAT( range, Catch::Matchers::RangeEquals(expected) )
with expansion:
  { 2 } elements are { 3 }

Actually get:

CHECK_THAT( range, Catch::Matchers::RangeEquals(expected) )
with expansion:
  { ? } elements are { 3 }