chore: fix typos

[code0xff]

This PR includes typo fixes.

---

Note

Renames istanbul backend method HasPropsal to HasProposal across interface, implementation, and usages, and fixes a comment typo in Block.Size.

• Consensus/Istanbul:
  • Rename HasPropsal -> HasProposal in consensus/istanbul/backend.go and implementation consensus/istanbul/backend/backend.go.
  • Update usage in IBFT core consensus/istanbul/ibft/core/preprepare.go.
  • Adjust test backend consensus/istanbul/ibft/core/testbackend_test.go accordingly.
• Core:
  • Fix comment typo in core/types/block.go for Block.Size ("previously" cached value).

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