Avoid duplicated sections when batching

[grarco]

In the function we use to construct a batch we push all the sections of an inner tx in the batch, even though some of these might already be present.

namada/crates/tx/src/types.rs

Lines 149 to 158 in 810c258

	pub fn add_inner_tx(&mut self, other: Tx, cmt: TxCommitments) -> bool {
	if !self.header.batch.insert(cmt) {
	return false;
	}
	// TODO: avoid duplicated sections to reduce the size of the message
	self.sections.extend(other.sections);
	true
	}

We should try to check if a section is already in the batch and, in this case, avoid pushing another identical one. This would lead to smaller transactions and a lower gas cost. In doing so, though, we should pay attention to the Commitments of that tx, which are computed on the salted sections: we might need to update the commitments to point to the equivalent section with a different salt.