Change BigInteger <-> BigUint conversions to use From instead of Into

[hdevalence]

Description

This is a strictly more general bound, and means that it's possible to write
the conversion without relying on type inference.

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• [ ] Wrote unit tests
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[Pratyush]

Thanks!

[weikengchen]

Let me try to fix the patching script. So I can get the "test against curves" passing.

[Pratyush]

@weikengchen a better way would be to temporarily add both git and version specifiers, like here: https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#multiple-locations

[Pratyush]

@hdevalence I pushed a fix for the CI issue in #324, and added the corresponding Fp impl in #325. If you could rebase then we can get this merged (I can't rebase because of the issue with "allow edits from maintainers")

[Pratyush]

@hdevalence if you could rebase/merge on top of the latest master, then we can get this merged, and can then merge #322 as well. Thanks!

[hdevalence]

@Pratyush thanks for the bump; I rebased this on top of current master.