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/serializer.hpp

@@ -256,7 +256,7 @@ class view_serializer
                 }
                 else
                 {
-                    write_float(float_value<number_float_t>(m_doc.str(n), n));
+                    write_float(float_value<number_float_t>(m_doc, n));
                 }
                 break;
             case value_t::object:    // LCOV_EXCL_LINE (containers are written by dump())