Add ownability trait

[xevisalle]

Resolves: #146

[moCello]

I think you can get away with only requiring a fn a function that returns the internal a.
owns and owns_unchecked can by computed from that, which means that the type that implements the Ownability trait doesn't need to re-compute the checks themselves.

Suggested change

	pub trait Ownability {
	/// Checks if:
	/// `note_pk ?= H(R · a) · G + B` for `ViewKey`
	/// `note_pk ?= (H(R · a) + b) · G` for `SecretKey`
	fn owns(&self, owner: &impl crate::Ownable) -> bool;
	/// Checks if `k_sync ?= R_sync · a`
	fn owns_unchecked(&self, owner: &impl crate::Ownable) -> bool;
	}
	pub trait Ownability {
	fn a(&self) -> JubJubScalar;
	fn owns(&self, owner: &impl crate::Ownable) -> bool {
	// implement owns here directly
	}
	fn owns_unchecked(&self, owner: &impl crate::Ownable) -> bool {
	// implement owns_unchecked here directly
	}
	}

[moCello]

With the Ownability trait we should also let the function note.decrypt_data (and with that also note.value and note.blinding_factor) to take any type that implements Ownability. This means that we can also decrypt the value and blinding factor with a SecretKey`

[xevisalle]

@moCello regarding your first comment: but then, how am I supposed to get b or B? In the end I still need two different ways to perform the check.

Regarding the decrypt data thing, I can look into it in a new commit.

[xevisalle]

Closing because Ownable is no longer there.