DRTVWR-564: Add LL::apply(): call function, passing args from tuple.

This anticipates C++17's std::apply(), and in fact once we detect C++17, we'll
just use that. But in C++14 we must still provide our own implementation.

indra/llcommon/apply.h

+/**
+ * @file   apply.h
+ * @author Nat Goodspeed
+ * @date   2022-06-18
+ * @brief  C++14 version of std::apply()
+ * 
+ * $LicenseInfo:firstyear=2022&license=viewerlgpl$
+ * Copyright (c) 2022, Linden Research, Inc.
+ * $/LicenseInfo$
+ */
+
+#if ! defined(LL_APPLY_H)
+#define LL_APPLY_H
+
+#include <tuple>
+
+namespace LL
+{
+
+#if __cplusplus >= 201703L
+
+// C++17 implementation
+using std::apply;
+
+#else // C++14
+
+// Derived from https://stackoverflow.com/a/20441189
+// and https://en.cppreference.com/w/cpp/utility/apply
+template <typename CALLABLE, typename TUPLE, std::size_t... I>
+auto apply_impl(CALLABLE&& func, TUPLE&& args, std::index_sequence<I...>)
+{
+    // call func(unpacked args)
+    return std::forward<CALLABLE>(func)(std::move(std::get<I>(args))...);
+}
+
+template <typename CALLABLE, typename... ARGS>
+auto apply(CALLABLE&& func, std::tuple<ARGS...>&& args)
+{
+    // std::index_sequence_for is the magic sauce here, generating an argument
+    // pack of indexes for each entry in args. apply_impl() can then pass
+    // those to std::get() to unpack args into individual arguments.
+    return apply_impl(std::forward<CALLABLE>(func),
+                      std::forward<std::tuple<ARGS...>>(args),
+                      std::index_sequence_for<ARGS...>{});
+}
+
+#endif // C++14
+
+} // namespace LL
+
+#endif /* ! defined(LL_APPLY_H) */