feat(zeroize): added zeorizing to Felt

[Trantorian1]

This is feature-gated behind the zeroing flag

Pull Request type

• Feature

What is the current behavior?

Felt provides no option to zeroize its memory, which can be an issue when dealing with sensitive cryptographic information such as private keys.

Resolves: #99

What is the new behavior?

• Implemented zeroizing for Felt through the zeroize crate. While zeroize is not used directly, Felt implements manually the Zeroize trait in the hopes of making this behavior more 'standard'. This has been gated behind the zeroing feature.

Note

Contrarily to zeroize, this is done using safe rust by leveraging core::mem::take to replace the in-memory representation of a felt with its default value of Felt::ZERO.

Does this introduce a breaking change?

No. While the zeroing feature has been added to default, zeroing has not been implemented on drop, both because this would interfere with the Copy trait of Felt but also because it seems like it should be up to the end user to determine when they are dealing with sensitive information.