Improve JSON serialization

[turbolent]

Description

Improve Serialize further:

• Also defensively escape <, >, & (XSS when metadata is rendered in a browser) and
  U+2028 / U+2029 (valid JSON but invalid in JS string literals)

• Improve efficiency by using StringBuilder

  All serialization now appends to a single shared StringBuilder instead of building and joining intermediate strings/arrays.

  Public functions keep their signatures and simply create a builder and delegate to private append* helpers.

• Add support for additional scalar types

  Added Fix64, Fix128, and UFix128 (previously only UFix64 was handled among fixed-point types).

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For contributor use:

• Targeted PR against main branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the standards mentioned here.
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[codecov-commenter]

Codecov Report

❌ Patch coverage is 99.12281% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cadence/contracts/utils/Serialize.cdc	99.12%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[SupunS]

Nice!

[janezpodhostnik]

Nice, thanks!