update `{next,prev}_consensus_state` implementations

[rnbguy]

Bug in next_consensus_state

The following comparison should be reversed.

basecoin-rs/basecoin/modules/src/ibc/client_contexts.rs

Lines 201 to 203 in 57f47f2

	if height > &IbcHeight::new(path.revision_number, path.revision_height).unwrap() {
	return Some(path);
	}

Refactor prev_consensus_state

We can simplify the prev_consensus_state as next_consensus_state if we reverse the iterator here.

basecoin-rs/basecoin/modules/src/ibc/client_contexts.rs

Line 233 in 57f47f2

let pos = keys.iter().position(|path| {

ref impl in ibc-rs testkit.

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Apart from that, next_consensus_state and prev_consensus_state - both cases, we are trying to find a partition point. Considering the keys are sorted, we may want to use partition_point.