contract: change return type of public_key() to String

[ppca]

Using String instead of AffinePoint for better readability

[ppca]

@volovyks With String return type, the public_key() fn call returns
"secp256k1:291ASHiiHRMCj1k6iVSvsFeYUBEvRNsk9TTWKfVw9wgkdGhWvcPHGMzPd2BZy2UVcobm9DfdKSM9GTeWAzNSjsWp"' in a local test, where if we return AffinePoint, the call will return something like ODxxxx, much less readable.
The user of the contract can choose to convert the String to whatever they'd like to use in their code

[ChaoticTempest]

we should make our own custom type here instead of using String which can be very arbitrary what type it is. That way, we can support similar forms of parsing in our testing

[ppca]

we should make our own custom type here instead of using String which can be very arbitrary what type it is. That way, we can support similar forms of parsing in our testing

@ChaoticTempest This PR was addressing #665, which I think aims at returning a universal type instead of our custom type so users find it easier as @volovyks told me.
I'm ok either ways tho.

[ChaoticTempest]

so our own custom type is actually the universal type, because we control what the semantics of it are. We need parsing logic at the end of the day anyways, so attaching it to our own custom type would be ideal because we can reuse it for testing and other users would be able to import it as well on Rust side of things