Convert the floats of json_view from the digit layout

The parser records where the integer digits, the fraction digits, and the
exponent of a float token are. For floats and doubles with at most 19
digits, the value is now read from that layout: the digits eight at a time,
without scanning the token, and rounded by the library's conversion core
(detail::decimal_to_float(): Clinger's fast path where both operands are
exact, else the Eisel-Lemire algorithm, which needs no fallback for up to 19
digits). It rounds correctly, so the values are those of parse(); other
tokens and types keep the library's conversion of the whole token.

get<double>(), materialize(), dump(), and comparisons use it. Traversing
canada.json (111,000 floats, every number converted): 0.95 -> 1.29 GB/s.

Tests add tokens around the limits (19 and 20 digits, 2^53, 10^22, and
those of float) to the bit-for-bit comparison with parse().

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

include/nlohmann/detail/view/materialize.hpp

@@ -81,7 +81,7 @@ BasicJsonType materialize(const document_data& d, const node* n)
                 ++n;
                 break;
             case value_t::number_float:
-                sax.number_float(float_value<typename BasicJsonType::number_float_t>(d.str(*n), *n), no_token);
+                sax.number_float(float_value<typename BasicJsonType::number_float_t>(d, *n), no_token);
                 ++n;
                 break;
             case value_t::boolean: