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Manually Implement SHA-256 Precompile Call (#359)

When looking for low-hanging fruit for gas optimisations, I noticed that
Solidity was generating copy loops when calling the SHA-256 precompile,
even if the input was already in memory!

This PR provides a work around where we re-implement the SHA-256
precompile manually in a way that it never copies the input before doing
the `staticcall`. This gives us some substantial gas savings, both in
terms of deployment costs and runtime costs:

<details><summary>Before:</summary>

```
  Gas Benchmarking [@bench]
    WebAuthnSigner
      ⛽ deployment: 515421
      ✔ Benchmark signer deployment cost (765ms)
      ⛽ verification (Dummy): 12106
      ✔ Benchmark signer verification cost with Dummy verifier
```

</details>

<details><summary>After:</summary>

```
  Gas Benchmarking [@bench]
    WebAuthnSigner
      ⛽ deployment: 471398
      ✔ Benchmark signer deployment cost (761ms)
      ⛽ verification (Dummy): 11148
      ✔ Benchmark signer verification cost with Dummy verifier
```

</details>

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