Add string.pack, string.packsize, string.unpack from Lua 5.3

[MCJack123]

This PR adds three new functions from Lua 5.3:

• string.pack (fmt, v1, v2, ...): Returns a binary string containing the values v1, v2, etc. packed (that is, serialized in binary form) according to the format string fmt.
• string.packsize (fmt): Returns the size of a string resulting from string.pack with the given format. The format string cannot have the variable-length options 's' or 'z'.
• string.unpack (fmt, s [, pos]): Returns the values packed in string s according to the format string fmt. An optional pos marks where to start reading in s (default is 1). After the read values, this function also returns the index of the first unread byte in s.

This passes the Lua 5.3.4 tests for the functions, with a few adjustments made regarding number sizes due to differences in Lua 5.1 & Java. I have included these tests in the PR as well.

The code is a bit messy right now and may need some refactoring to look decent, but it works.

[MCJack123]

Oops, forgot to apply the endianness/alignment changes everywhere. This is now fixed. I also switched unpack to use ArrayLists.