code: Cleanup various proposal-related types

[romac]

We have somehow ended up with several different types with similar names holding similar but not identical information, which is quite confusing for everybody (me included).

Let's review those and see if we can deduplicate them or find better names for each of them:

malachite/code/crates/core-consensus/src/types.rs

Lines 44 to 50 in 55fc086

	pub struct ValueToPropose<Ctx: Context> {
	pub height: Ctx::Height,
	pub round: Round,
	pub valid_round: Round,
	pub value: Ctx::Value,
	pub extension: Option<SignedExtension<Ctx>>,
	}

malachite/code/crates/core-consensus/src/types.rs

Lines 54 to 62 in 55fc086

	pub struct ProposedValue<Ctx: Context> {
	pub height: Ctx::Height,
	pub round: Round,
	pub valid_round: Round,
	pub proposer: Ctx::Address,
	pub value: Ctx::Value,
	pub validity: Validity,
	pub extension: Option<SignedExtension<Ctx>>,
	}

malachite/code/crates/engine/src/host.rs

Lines 22 to 27 in 55fc086

	pub struct LocallyProposedValue<Ctx: Context> {
	pub height: Ctx::Height,
	pub round: Round,
	pub value: Ctx::Value,
	pub extension: Option<SignedExtension<Ctx>>,
	}

There is also the Proposal trait, which is not actually implemented for any of those, but only for the concrete proposal type, which does not help with the confusion:

malachite/code/crates/core-types/src/proposal.rs

Lines 6 to 28 in 55fc086

	pub trait Proposal<Ctx>
	where
	Self: Clone + Debug + Eq + Send + Sync + 'static,
	Ctx: Context,
	{
	/// The height for which the proposal is for.
	fn height(&self) -> Ctx::Height;
	/// The round for which the proposal is for.
	fn round(&self) -> Round;
	/// The value that is proposed.
	fn value(&self) -> &Ctx::Value;
	/// The value that is proposed.
	fn take_value(self) -> Ctx::Value;
	/// The POL round for which the proposal is for.
	fn pol_round(&self) -> Round;
	/// Address of the validator who issued this proposal
	fn validator_address(&self) -> &Ctx::Address;
	}

[cason]

From the consensus point of view, we have a Value that is anything that is proposed and decided in consensus. I am not sure whether the extensions in the code are decided or just data exchanged "outside consensus" (not committed).

A Proposal is a Value with the metadata needed to the first round step of Tendermint. Namely, height, round, valid round, and some identification of the proposer process (can be just a signature).

[cason]

I see some relation with #365.

It is not 100% clear to me whether the Proposal from the Tendermint protocol carries a "full" Value or an unique identifier of the proposed "full" Value. On Quint this is trivial, as id(v) = v, but implementation-wise this is super relevant.