Fix the compiler and lint failures of the json_view comparison

- all_digits(): the lambda declares its constant itself; MSVC does not
  let a lambda without captures read a constexpr local of the function
- equal(): the frame's "in document order" index is a namespace-scope
  constant (document_order); the local constexpr npos in the default
  member initializers of the local struct crashed GCC 9 and MinGW GCC
  and was not found by MSVC
- keys_differ_early() keeps the first keys in std::arrays (cpplint:
  variable-length arrays; clang-tidy: C arrays), and the basic_json
  side's first_keys() is not noexcept (key() may throw)
- the numbers of one type are compared in an if chain instead of a
  nested conditional operator
- the dump test avoids C arrays and temporary string concatenations

View comparison instructions are unchanged (Apple clang, twitter, citm,
canada: -0.1 % to +0.1 %).

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

include/nlohmann/detail/view/image.hpp

@@ -282,9 +282,9 @@ inline const unsigned char* digit_run(const unsigned char* p, const unsigned cha
 /// whether the n bytes at p are ASCII digits (n > 0)
 inline bool all_digits(const unsigned char* p, std::size_t n) noexcept
 {
-    constexpr std::uint64_t ones = 0x0101010101010101ull;
     const auto digits = [](const unsigned char* q)
     {
+        constexpr std::uint64_t ones = 0x0101010101010101ull;
         const std::uint64_t x = read_eight_bytes(q) ^ (ones * 0x30u);
         return (((x + (ones * 0x76u)) | x) & (ones * 0x80u)) == 0;
     };