Add iterator+sentinel tests and docs for binary deserializers

This commit extends the C++20 ranges support (iterator+sentinel pairs) to the
binary format deserializers from_cbor, from_msgpack, from_ubjson, from_bjdata,
and from_bson, matching what was already done for parse(), accept(), and
sax_parse().

Changes:
- Add istreambuf_sentinel helper to test_utils.hpp for EOF detection in tests
- Add 5 new test cases that read binary files directly via
  std::istreambuf_iterator<char> + sentinel, without pre-buffering
- Update documentation for all 5 from_* functions to document overload (3)
  with SentinelType parameter
- All tests pass; verified against existing test suite data
- Fix potential buffer over-read warning in heterogeneous iterator test

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/test_utils.hpp

@@ -30,4 +30,20 @@ inline std::vector<std::uint8_t> read_binary_file(const std::string& filename)
     return byte_vector;
 }
 
+// sentinel for istreambuf_iterator; compares != true until EOF is reached
+// lets tests read a file directly via the new iterator+sentinel overloads
+// instead of buffering the whole file into a vector first
+struct istreambuf_sentinel
+{
+    friend bool operator!=(const std::istreambuf_iterator<char>& it, const istreambuf_sentinel&) noexcept
+    {
+        return it != std::istreambuf_iterator<char>();
+    }
+
+    friend bool operator!=(const istreambuf_sentinel&, const std::istreambuf_iterator<char>& it) noexcept
+    {
+        return it != std::istreambuf_iterator<char>();
+    }
+};
+
 } // namespace utils