Address the review of the view builder and document

Changes promised in the replies to the review of #5620:

- read() reuses the owned copy: a copied input (an rvalue byte container,
  a stream, FILE*, wide characters, an iterator range) is appended to the
  cleared owned buffer instead of a fresh std::string being move-assigned
  over it. A moved-in std::string still replaces it.
- The error replay runs the library parser with a SAX handler that stores
  nothing and throws the reported exception, instead of
  BasicJsonType::parse(), which built a DOM up to the error.
- Pointers to and arrays of wide characters, and FILE*, take the input
  adapter path (converted to UTF-8 into an owned buffer) instead of
  failing a static_assert. Wide pointers are NUL-terminated.
- Streams are read in blocks through rdbuf() into the buffer, instead of
  character by character through the input adapter; eofbit is set at the
  end, as by the library's stream adapter.
- tape/tape_size/tape_cap are now nodes/node_count/node_capacity.
- The builder's members get descriptive, m_-prefixed names.
- The shallow frame stack is a std::array (in a wrapper with a
  user-provided constructor: -Weffc++ wants the member initialized, and
  value-initializing the 64 frames costs up to 30 % on small documents).
- The value dispatch macro has no NEXT parameter; the switch is left with
  break, and the macro is undefined after its last use.
- literal() and digits_exceed() deduce the length from the string literal;
  each literal has one helper; the two `if (!is_float)` blocks are merged.
- arena_ensure() sits right before the writes; the decode arena is a
  std::vector<char>.
- The builder records the deepest nesting, and materialize() sizes its
  stack from it once (growing it only if a subtree is deeper).
- Comment fixes (hex4, slow_string, the NUL rule, put_utf8, 0 - m, the
  arena resize, grow()) and formatting (macro backslashes, digit cases,
  one power of ten per line, value_t assertion next to is_container()).
- build_with's template parameters follow the builder's order.

Greg's idea of keeping the source offset in `off` and the arena offset in
`next` for escaped strings was measured and not adopted: string access got
7-18 % slower with a select, and 3.5-18 % slower with a branch-free field
pick (Apple M-series, clang -O3), parsing was unchanged.

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

include/nlohmann/detail/view/node.hpp

@@ -21,14 +21,6 @@ namespace detail
 namespace view
 {
 
-// the node kinds are value_t values; the tests of is_container() and of the
-// number kinds depend on this numbering
-static_assert(static_cast<std::uint8_t>(value_t::null) == 0 && static_cast<std::uint8_t>(value_t::object) == 1
-              && static_cast<std::uint8_t>(value_t::array) == 2 && static_cast<std::uint8_t>(value_t::string) == 3
-              && static_cast<std::uint8_t>(value_t::boolean) == 4 && static_cast<std::uint8_t>(value_t::number_integer) == 5
-              && static_cast<std::uint8_t>(value_t::number_unsigned) == 6 && static_cast<std::uint8_t>(value_t::number_float) == 7,
-              "the node format depends on the numbering of value_t");
-
 /// The largest input a document accepts, in bytes. Offsets and node counts are
 /// 32 bits wide; the limit keeps 16 bytes (the width of the scanner's steps)
 /// below 2^32, so that a position one step past the end of the text fits.
@@ -60,6 +52,14 @@ struct node
 };
 static_assert(sizeof(node) == 16, "node must stay 16 bytes");
 
+// the node kinds are value_t values; the test below (object and array are 1
+// and 2) and those of the number kinds depend on this numbering
+static_assert(static_cast<std::uint8_t>(value_t::null) == 0 && static_cast<std::uint8_t>(value_t::object) == 1
+              && static_cast<std::uint8_t>(value_t::array) == 2 && static_cast<std::uint8_t>(value_t::string) == 3
+              && static_cast<std::uint8_t>(value_t::boolean) == 4 && static_cast<std::uint8_t>(value_t::number_integer) == 5
+              && static_cast<std::uint8_t>(value_t::number_unsigned) == 6 && static_cast<std::uint8_t>(value_t::number_float) == 7,
+              "the node format depends on the numbering of value_t");
+
 NLOHMANN_VIEW_ALWAYS_INLINE bool is_container(const node& n) noexcept
 {
     return static_cast<unsigned>(n.kind) - 1u <= 1u;