Use 16kib leaf sizes when calculating code roots

[Voxelot]

Spec change: FuelLabs/fuel-specs#476

Improve the performance of calculating code roots by using a larger leaf size in the merkle tree to 16kib.

• Chunk bytecode and predicate code by 16kib chunks.
• Zero pad chunks out to the nearest multiple of 8 bytes if they are < 16kib. For example: if a predicate is only 500bytes, it should be padded out to 504 bytes no 16 kib.

[freesig]

@Voxelot What is the purpose of the padding?

[bvrooman]

Zero pad chunks out to the nearest multiple of 8 bytes if they are < 16kib. For example: if a predicate is only 500bytes, it should be padded out to 508 bytes no 16 kib.

To be clear: 500 is already not divisible by 8; i.e. 500 mod 8 = 4. Do you mean that 500 bytes would be zero padded to 504 bytes, since 504 mod 8 = 0?

[Voxelot]

@Voxelot What is the purpose of the padding?

To preserve the original padding we used of 8 bytes. This is likely somewhat arbitrary though.