Wrapper for BigInt type

[austinabell]

Would be ideal to have a wrapper for our own defined Big int type for a couple reasons:

1. To apply common functionality on the type (like converting to bytes, encoding in a specific way, or converting to other used types)
2. If we intend to change the dependency or implementation for this, it only has to be changed in one place instead of changing everywhere.

It's just starting to get annoying to use so I am suggesting getting ahead of the curve on this one, let me know if you guys agree.

Currently the dep we are using throughout the codebase is num-bigint = "0.2.3" but would be nice to just have a crate (probably at root level) to be used.

[austinabell]

Don't really see this as adding any value as all functions needed for Filecoin stuff is implemented in this type it seems (but can revisit later)