fix signing payloads > 256 bytes

[clangenb]

Closes #418

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (653d39a) 48.85% compared to head (ba547c6) 48.84%.
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #419      +/-   ##
==========================================
- Coverage   48.85%   48.84%   -0.02%     
==========================================
  Files         177      177              
  Lines        9443     9445       +2     
==========================================
  Hits         4613     4613              
- Misses       4830     4832       +2     

Flag	Coverage Δ
polkadart	16.82% <0.00%> (-0.03%)	⬇️
polkadart_cli	12.04% <ø> (ø)
polkadart_keyring	71.76% <ø> (ø)
polkadart_scale_codec	54.93% <ø> (ø)
secp256k1_ecdsa	90.00% <ø> (ø)
sr25519	85.91% <ø> (ø)
ss58	96.87% <ø> (ø)
substrate_bip39	56.37% <ø> (ø)
substrate_metadata	87.61% <ø> (ø)

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Files	Coverage Δ
...kages/polkadart/lib/extrinsic/signing_payload.dart	0.00% <0.00%> (ø)