json conversion of some float64 values seems less precise than expected

[spoonincode]

Consider the following which uses the fpconv_dtoa() function as used by abieos' bin->json conversion,

int main() {
	double x = 151115727451828646838272.0;
	std::cout << std::to_string(x) << std::endl;

	char buf[26] = {};
        fpconv_dtoa(x, buf);
	std::cout << buf << std::endl;
	return 0;
}

The output is,

151115727451828646838272.000000
151115727451828650000000

This value is 2^77 and should be perfectly representable in a double -- and std::to_string() clearly shows it as being so. But when fpconv_dtoa() prints it some precision is lost.