Fix 64-bit upper half power

[deathcap]

Bitwise operators unfortunately only operate on 32-bit integers, so x<<32 === x, and

        return (upper << 32) + lower;

is equivalent to upper + lower. For example this NBT file: https://github.com/deathcap/playerdat/blob/master/test.dat parses as:

"firstPlayed": 2016156000

This PR changes the calculation to:

        return (upper * 4294967296) + lower;

so it can use the full range of 53-bit floating point, giving the correct value:

"firstPlayed": 1367815755810

(cross checked with NBTExplorer and max-mapper/minecraft-nbt#2)

Precision loss can still occur (above 2^53 = 9007199254740992), so this doesn't fix GH-1 completely, but it takes care of some common cases.

[deathcap]

The tests fail since the double is rounded to -9151314442816848000, can't precisely represent the expected value -9151314442816847872

[deathcap]

Closing; better fix (with passing tests) in #9