Address the review of the json_view object index and SIMD scan

From gregmarr's review of #5628:

- Docs and comments say that the hash index is one of the object's keys.
- object_index.hpp: conditional_static_cast for the hash, "slot" for the
  table position, no flag for a degenerate table, and a static_assert
  that the smallest indexed object is larger than the probe window.
- scan.hpp unrolls with NLOHMANN_VIEW_REPEAT_FIRST_8/SECOND_8.

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

include/nlohmann/detail/view/node.hpp

@@ -45,7 +45,7 @@ struct node
 {
     std::uint8_t kind;   ///< value_t
     std::uint8_t flags;  ///< node_flags
-    std::uint16_t extra; ///< numbers: integer digits (low byte) and fraction digits (high byte), 255 = "many"; objects: number of the hash index (1-based, 0 = none); otherwise 0
+    std::uint16_t extra; ///< numbers: integer digits (low byte) and fraction digits (high byte), 255 = "many"; objects: number of the object's key hash index (1-based, 0 = none); otherwise 0
     std::uint32_t off;   ///< source offset (string content, number token, literal, bracket); arena offset if node_flags::escaped
     std::uint32_t len;   ///< string: decoded bytes; float: token bytes; array/object: element count
     std::uint32_t next;  ///< array/object: number of nodes of the subtree (its extent in the enclosing sequence)