Avoid heap allocation for function calls with a small number of args (#5824)

* Avoid heap allocation for function calls with a small number of arguments

We don't have access to llvm::SmallVector or similar, but given the
limited subset of the `std::vector` API that
`function_call::args{,_convert}` need and the "reserve-then-fill"
usage pattern, it is relatively straightforward to implement custom
containers that get the job done.

Seems to improves time to call the collatz function in
pybind/pybind11_benchmark significantly; numbers are a little noisy
but there's a clear improvement from "about 60 ns per call" to "about
45 ns per call" on my machine (M4 Max Mac), as measured with
`timeit.repeat('collatz(4)', 'from pybind11_benchmark import
collatz')`.

* clang-tidy

* more clang-tidy

* clang-tidy NOLINTBEGIN/END instead of NOLINTNEXTLINE

* forgot to increase inline size after removing std::variant

* constexpr arg_vector_small_size, use move instead of swap to hopefully clarify second_pass_convert

* rename test_embed to test_low_level

* rename test_low_level to test_with_catch

* Be careful to NOINLINE slow paths

* rename array/vector members to iarray/hvector. Move comment per request. Add static_asserts for our untagged union implementation per request.

* drop is_standard_layout assertions; see https://github.com/pybind/pybind11/pull/5824#issuecomment-3308616072

tests/test_with_catch/catch.cpp

@@ -0,0 +1,43 @@
+// The Catch implementation is compiled here. This is a standalone
+// translation unit to avoid recompiling it for every test change.
+
+#include <pybind11/embed.h>
+
+// Silence MSVC C++17 deprecation warning from Catch regarding std::uncaught_exceptions (up to
+// catch 2.0.1; this should be fixed in the next catch release after 2.0.1).
+PYBIND11_WARNING_DISABLE_MSVC(4996)
+
+// Catch uses _ internally, which breaks gettext style defines
+#ifdef _
+#    undef _
+#endif
+
+#define CATCH_CONFIG_RUNNER
+#include <catch.hpp>
+
+namespace py = pybind11;
+
+int main(int argc, char *argv[]) {
+    // Setup for TEST_CASE in test_interpreter.cpp, tagging on a large random number:
+    std::string updated_pythonpath("pybind11_test_with_catch_PYTHONPATH_2099743835476552");
+    const char *preexisting_pythonpath = getenv("PYTHONPATH");
+    if (preexisting_pythonpath != nullptr) {
+#if defined(_WIN32)
+        updated_pythonpath += ';';
+#else
+        updated_pythonpath += ':';
+#endif
+        updated_pythonpath += preexisting_pythonpath;
+    }
+#if defined(_WIN32)
+    _putenv_s("PYTHONPATH", updated_pythonpath.c_str());
+#else
+    setenv("PYTHONPATH", updated_pythonpath.c_str(), /*replace=*/1);
+#endif
+
+    py::scoped_interpreter guard{};
+
+    auto result = Catch::Session().run(argc, argv);
+
+    return result < 0xff ? result : 0xff;
+}