feat!: implementing BignumTrait for each type using metaprogramming

[kashbrti]

Description

We move the BigNumTrait impls into a compttime macro.
For each BigNum type that we construct, we call this macro to implement all the functions in the trait
with the correct generics.

Problem*

Resolves
#142

Summary*

Additional Context

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[kashbrti]

@TomAFrench, this PR will have significant changes to the interface. Should I update the ReadMe accordingly?

[TomAFrench]

Yeah, we'll need to update the readme to account for these changes. Please do.

[TomAFrench]

I've made a few cleanup changes in the last few commits:

• BigNum struct is removed and BigNumTrait is renamed to BigNum to be less tautological.
• The BigNumParamsGetter trait is removed along with all of the unit structs which implement it.
• I've shifted some of the implementations of bignum trait methods into the definition so we can just have a default implementation rather than relying on metaprogramming for everything.

[kashbrti]

@TomAFrench I'm updating the paramgen as well to work with this version of the lib