Rename unwrap in k256_serde and remove panics

[milapsheth]

• Added AsRef for k256_serde wrappers to expose the k256 structs
• Switched from unwrap() to as_ref()
• Removed various potential panics
• Switched from assert! to debug_assert! Fixes #121 Fixes #122

[milapsheth]

Creating another PR since this the workflow is broken here