Append ExtraVanity to the beginning of the marshaled ExtraData

[Stefan-Ethernal]

Description

This PR appends ExtraVanity bytes to the beginning of marshalled extra data bytes slice in the MarshalRLPTo function, instead of doing so for each occasion MarshalRLPTo gets invoked.

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Breaking changes

Please complete this section if any breaking changes have been made, otherwise delete it

Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually

Manual tests

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Documentation update

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Additional comments

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[vcastellm]

LGTM