Remove custom Serde implementation

[daxpedda]

Based on #42 and #43 we don't really need a custom implementation for Serde anymore.

Advantages:

• Less code to maintain on our side.
• No weird where bounds for consumers using Serde instead of our de/serialize functions.
• Better Serde implementation, instead of calling our de/serialize function, which does manual de/serialization.

Disadvantages:

• Cargo's weak dependency feature isn't stable yet, consumers will have to manually activate the serde crate feature for the curve25519-dalek or p256 dependency. Not that big of a deal considering we aren't re-exporting them anyway, consumers will have to pull in these crates manually anyway.