Add support for negative integer encoding/decoding.

[JyotinderSingh]

Presently dencoding/int.go only supports unsigned integers.

• Add support for signed integer encoding, this would allow us to support signed integer stacks and queues down the line.
• Add unit tests to validate correctness.
• Benchmark the code and measure the time taken and memory allocs using benchmem and try to keep them to the bare minimum.

[Vsareen0]

Hey @JyotinderSingh if no one has picked it, I would like to give this a go. !

[JyotinderSingh]

Hey @JyotinderSingh if no one has picked it, I would like to give this a go. !

Assigned

[JyotinderSingh]

Fixed in #216