Add `ERC721ABatchTransferable` and `ERC721ABatchBurnable`

[Vectorized]

Continued from #458 and #444.

Notes:

• Refactored some stuff to return uint256 instead of address.
• Did some minor optimizations. Changing == 0 to == uint256(0) helps the compiler produce more efficient bytecode.
• Doing an array access will incur a bounds check. Thus we use assembly to directly read the memory array values.
• Even though the assembly helper methods are private, we'll give them a uncommon name to prevent collision. Private methods can collide in namespace.

[Nlferu]

This is huuuge!!! Thanks a lot @jjranalli @Vectorized @chiru-labs @cygaar amazing job 🔥🔥🔥