Fix fetch_previous_block_hashes going out of bounds

[cpubot]

The current implementation attempts to access an out of bounds index when block number is > 255.
In particular, assume block 256 is given.

let start = block_number.saturating_sub(256);
let futs: FuturesOrdered<_> = (start..=block_number)
    .rev()
    .step_by(2)
    .map(|block_number| Self::fetch(rpc_url, block_number))
    .collect();

129 responses are returned.

Later, the responses are iterated over:

for (idx, response) in responses.iter().skip(1).enumerate() {
    // ...
    hashes[starting_offset - 2 * idx] = response.result.hash;
}

Assume starting_offset = 254. When idx reaches 128, starting_offset - 2 * idx == -2.

Assume starting_offset = 253. When idx reaches 127, starting_offset - 2 * idx == -1.

The new implementation provided here constructs the Vec of hashes iteratively, simplifying the logic and avoiding any indexing issues. The solution was tested and verified to be functionally equivalent to the current solution for blocks 0-255.

[Nashtare]

Good catch, thanks for fixing it!