Make the value lifecycle inline-friendly (#5840)

* Make the value lifecycle inline-friendly

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

* Compare standard containers without a thread-local depth count

operator== on objects and arrays recursed through the containers' own
operator== and counted the depth in a thread-local variable. Where reading a
thread-local costs a call (macOS), that added about 10% to comparing documents
with many small containers. For the standard containers, walk objects and
arrays directly, compare scalars and strings inline, and pass the depth as an
argument, falling back to compare_iteratively beyond the depth limit. Other
container types keep the previous path.

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

* Address review: owns_memory() helper, leaf compare naming, astyle

- Add detail::owns_memory(value_t), a switch listing every enumerator so
  -Wswitch-enum flags it when a type is added; destroy() and the copy
  constructor use it instead of hand-written comparisons. Clang emits the
  same bitmask test as before.
- Drop the parentheses around the scalar operands in
  JSON_IMPLEMENT_SCALAR_CASES; they are only needed for the container
  operands (#1530), which the comment now says.
- Build equal_bounded_containers from detail::conjunction so astyle no
  longer pads the template brackets.
- Rename equal_scalar() to equal_leaf() and document why it also compares
  strings, unlike JSON_IMPLEMENT_SCALAR_FAST.

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

---------

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

include/nlohmann/detail/value_t.hpp

@@ -117,6 +117,34 @@ inline bool operator<(const value_t lhs, const value_t rhs) noexcept
 }
 #endif
 
+/*!
+@brief whether a value of the given type owns heap memory
+
+Strings, binaries, objects, and arrays are stored as pointers to allocated
+values; all other types are stored directly in the union. Every enumerator is
+listed so that -Wswitch-enum flags this function when a new type is added.
+*/
+inline bool owns_memory(const value_t t) noexcept
+{
+    switch (t)
+    {
+        case value_t::object:
+        case value_t::array:
+        case value_t::string:
+        case value_t::binary:
+            return true;
+
+        case value_t::null:
+        case value_t::boolean:
+        case value_t::number_integer:
+        case value_t::number_unsigned:
+        case value_t::number_float:
+        case value_t::discarded:
+        default:
+            return false;
+    }
+}
+
 
 /*!
 @brief compare an integer with a floating point number without precision loss