offset overflow on 32-bit systems

[williambrode]

Working with the code I noticed this:

type Chunker struct {
...
	offset int

Where offset is tracking the offset of this chunk inside the file. On a 32-bit system, offset would be limited to 2GB and would then overflow. This is problematic when dealing with large files. offset should be an int64 to prevent this issue.

[williambrode]

I should mention the Chunk struct has the same field with the same issue.