Light Client minor improvements

[ebuchman]

Follow up to #110 Types:

• memoize results of methods that don't take args, eg. .hash() (Light client impl followup  #96 (comment))
• introduce a simple event type and let bisection function take a closure that will fire events. this can be used for logging so we don't need a print statement or logger inside the functions

Also follow up to PR review:

• Simplify some light client types and testing #132 (review)

[ebuchman]

Possibly break this up into two issues. Memoizing isn't important for https://github.com/informalsystems/tendermint-rs/milestone/1 but some good form of logging probably is

[brapse]

It isn't clear to me what the motivation or expectation is here.

Is it possible that we want to know what blocks were part of a verification path? In which case this might be fulfilled by the tracing feature which is already landed:

tendermint-rs/light-client/src/state.rs

Lines 32 to 42 in 740cb66

	/// Record that the block at `height` was needed to verify the block at `target_height`.
	///
	/// ## Preconditions
	/// - `height` < `target_height`
	#[pre(height <= target_height)]
	pub fn trace_block(&mut self, target_height: Height, height: Height) {
	self.verification_trace
	.entry(target_height)
	.or_insert_with(HashSet::new)
	.insert(height);
	}

If not, it would be great to update the description of this issue with what we want and why.

[ebuchman]

I think we can close for now