Fix strtod

[abolz]

Fixes #849, #1249, #1251, #1253, #1256, #1259

The issues were mainly because of unsigned integer overflow and boundary cases. Switched to signed integer arithmetic.

To do:

• This patch currently requires "small inputs" and "small exponents" due to integer overflow. Fixing this probably requires some more changes in ParseNumber. (Or simply using int64 instead of int32)
• Some inaccuracies in full-precision mode (due to discarding non-zero bits) should be fixed.

[miloyip]

Really thank for your contribution.
There is some valgrind errors such as https://travis-ci.org/Tencent/rapidjson/jobs/392709049#L2555 Can you take a look?

[abolz]

Yes, looking into it. But honestly I have no idea whats wrong... Valgrind complains about an uninitialized variable of size 4 in StrtodBigInteger.

[abolz]

BTW could you cancel all the running appveyor builds?

[coveralls]

Coverage increased (+0.0003%) to 99.921% when pulling 7101911 on abolz:fix-strtod into 01c7174 on Tencent:master.

[abolz]

The valgrind error is fixed now (was due to an incorrect offset computation in BigInteger operator<<).
There are still some tests for edge cases in full-precision mode failing, which need to be fixed.

[miloyip]

Thank you for your precious work!