[Compiler] Improve default functions existence with conditions

[SupunS]

Work towards #3742

Description

Improves the compilation (desugaring in particular), to support co-existence of default functions and function conditions.
Similar to pre/post conditions, default functions are also flattened.

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• Targeted PR against master 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

[turbolent]

Great work!

[github-actions]

Cadence Benchstat comparison

This branch with compared with the base branch onflow:feature/compiler commit 9de8b2b
The command for i in {1..N}; do go test ./... -run=XXX -bench=. -benchmem -shuffle=on; done was used.
Bench tests were run a total of 7 times on each branch.

Collapsed results for better readability

[SupunS]

@turbolent Can you maybe have another look? Particularly the last two commits 🙏