Release v0.3.0 / Re-export num_traits::{One, Zero} from ark-std

[weikengchen]

Description

This PR re-exports num_traits::{One, Zero} from ark-std as well as prepares this repo for a release to crates.io.

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Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (master)
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

N/A:

• 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

[weikengchen]

It passes the checks, but let us be careful with this one (it is too important!).

If this PR looks good and is ready, one with write permission can (1) merge it and (2) add a GitHub tagged release.

[weikengchen]

(I don't have write access and would need someone with it to merge and also do cargo release)