Use the shared recursion limit in update() and merge_patch()

merge_depth_limit() is gone in favor of detail::recursion_depth_limit().
The two identical function-local frame structs become one member struct,
merge_frame, with a constructor, so both loops emplace_back() their
frames. merge_patch_iteratively() copies the frame it works on out of the
stack and changes it only through stack.back().

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

tests/src/unit-modifiers.cpp

@@ -1041,7 +1041,7 @@ TEST_CASE("update() on deeply nested values")
     SECTION("merging past the descent bound gives the same result")
     {
         // every depth on either side of where the iterative version takes
-        // over (basic_json::merge_depth_limit(), 128)
+        // over (detail::recursion_depth_limit(), 128)
         for (std::size_t depth = 0; depth <= 300; ++depth)
         {
             CAPTURE(depth);