chore(Data/Nat/Size): simplify proofs, reduce dependencies

[astrainfinita]

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[github-actions]

PR summary de4f458186

Import changes for modified files

Dependency changes

File	Base Count	Head Count	Change
Mathlib.Data.Nat.Size	123	68	-55 (-44.72%)

Import changes for all files

Files	Import difference
Mathlib.Data.Nat.Size	-55
Mathlib.Data.FP.Basic	-1

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Declarations diff

+ shiftLeft'_true_eq_mul_pow
+ shiftLeft'_true_ne_zero

You can run this locally as follows

## summary with just the declaration names:
./scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh contains some details about this script.

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No changes to technical debt.

You can run this locally as

./scripts/reporting/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).