Add editable json_documents: set, push_back, insert, and erase

basic_json_document gets a second template parameter, Editable
(false by default), plus the aliases json_editable_document,
json_editable_view, ordered_json_editable_document and
ordered_json_editable_view.

Editable documents can change values and structure without
rewriting the source text: set()/push_back() on values, keys,
array indices and JSON pointers; insert() before an array
element; erase() of an object key, array index or JSON pointer.

New values and element sequences go into edit storage that the
document owns and never moves, so views keep referring to their
value across edits and a parsed node never moves. Read-only
documents walk the plain node array and are unaffected.

Strings are checked for UTF-8 on entry, so dump() of an editable
document never throws type_error.316. Binary values cannot be
stored (type_error.319).

A seeded differential test applies random edits to an editable
document and to the equivalent ordered_json and compares both
after every step.

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

include/nlohmann/detail/view/number.hpp

@@ -10,6 +10,7 @@
 
 #include <cstddef> // size_t
 #include <cstdint> // int64_t, uint64_t
+#include <limits> // numeric_limits
 #include <string> // string
 #include <type_traits> // integral_constant
 
@@ -128,11 +129,31 @@ NLOHMANN_VIEW_ALWAYS_INLINE FloatType layout_float(const unsigned char* p, const
     return decimal_to_float<FloatType>(layout_decimal(p, e, int_digits, frac_digits, limit));
 }
 
+/// the value of a float set by an edit: its token (the shortest round-trip
+/// text, or "nan", "inf", "-inf") in the edit arena
+template<typename FloatType>
+NLOHMANN_VIEW_NOINLINE FloatType edited_float(const char* token, const node& n)
+{
+    if (token[0] == 'n')
+    {
+        return std::numeric_limits<FloatType>::quiet_NaN();
+    }
+    if (token[0] == 'i' || (token[0] == '-' && token[1] == 'i'))
+    {
+        return token[0] == 'i' ? std::numeric_limits<FloatType>::infinity() : -std::numeric_limits<FloatType>::infinity();
+    }
+    return float_value<FloatType>(token, n);
+}
+
 /// the value of the float token of a node, as parse() converts it; floats and
 /// doubles with at most 19 digits are converted from the digit layout
 template<typename FloatType>
 FloatType float_value(const document_data& d, const node& n)
 {
+    if (NLOHMANN_VIEW_UNLIKELY((n.flags & node_flags::storage) == node_flags::edited))
+    {
+        return edited_float<FloatType>(d.str(n), n);
+    }
     return float_value<FloatType>(d, n, std::integral_constant<bool, has_native_float_format<FloatType>::value> {});
 }