Fix block height encoding for heights 1-16

[teor2345]

The Zcash mainnet and testnet chains encode heights 1 to 16 (inclusive) as individual numeric opcodes (OP_N). These opcodes are encoded as n + 0x50.

But the spec requires a "1-byte signed little endian" encoding:

Here is the coinbase field of mainnet block 1, showing the OP_1 encoding:

{
  "hash": "0007bc227e1c57a4a70e237cad00e7b7ce565155ab49166bc57397a26d339283",
  "height": 1,
  "tx": [
    {
      "txid": "851bf6fbf7a976327817c738c489d7fa657752445430922d94c983c0b9ed4609",
      "version": 1,
      "vin": [
        {
          "coinbase": "5100",
        }
      ],
    },
  ],
}

(I removed all the irrelevant fields.)

Here is an example of correct deserialization:

// Blocks 1 through 16 inclusive encode block height with OP_N opcodes.
(Some(op_n @ 0x51..=0x60), len) if len >= 1 => Height((op_n - 0x50) as u32),

Based on https://github.com/ZcashFoundation/zebra/blob/ebe1c9f88ecc44570ee335c3e8a25bcb53672acd/zebra-chain/src/transparent/serialize.rs#L55-L60

[daira]

Thanks, I'll fix it.