Add support for 448-bit integers and fields

[daira]

Description

Add support for 448-bit integers and fields. These will be needed for the Pluto and Eris curves
(https://github.com/daira/pluto-eris).

The changes are trivial and amount to applying the existing macros to a length of 448 bits (7 64-bit words).
This interacts with #186, but it should be easy to resolve any conflict.

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• Targeted PR against correct branch (master)
• Linked to Github issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code [not needed]
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

[Pratyush]

Thanks for the PR!

[daira]

Changing this to draft while I check that it actually works for Pluto/Eris (there seems to be some minor problem with SquareRootField, so perhaps I missed something).

[daira]

I didn't miss anything. When testing with the Pluto/Eris implementation I just needed to patch ark-ff and ark-serialize to use the 0.2.0 releases plus this patch only. I.e. in Cargo.toml:

[patch.crates-io]
ark-ff = { path = '../algebra/ff' }
ark-serialize = { path = '../algebra/serialize' }

And then it works. So, I think this PR is fine. (I rebased it relative to the 0.2.0 release for more straightforward testing, and removed the CHANGELOG.md entry because that would conflict.)

[ValarDragon]

LGTM!