Local bigint serialization

[austinabell]

Summary of changes
Changes introduced in this pull request:

• Implemented local serialization for big int types
• Updated dependency references from using my bigint fork

So since the serde macros don't call the associated variant of serialize, you can use a module in place of the type to serialize (similar to how serde_bytes works). This is important in future to not have to deploy/use a forked bigint dependency, cleaner for prod and makes easier to update dependency in the future. I have all bigint dependencies routing through this crate I created.

Downside to this is having to define a tuple type to annotate the serialization method. Another option is to later add a type to be able to convert to/from which could act as a proxy for serialization similar to how it is done currently when types need to be converted for serialization (I think the usage added in this PR is cleaner though).

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