refactor(Analysis): golf Mathlib/Analysis/Normed/Operator/Mul

[yuanyi-350]

• refactors Normed/Operator/Mul by defining ring_lmap_equiv_selfₗ as the symmetry of ContinuousLinearMap.toSpanSingletonLE
• shortens the norm proof for ring_lmap_equiv_self to ContinuousLinearMap.norm_toSpanSingleton

Extracted from #37968

[github-actions]

PR summary c3e588b468

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

No declarations were harmed in the making of this PR! 🐙

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.

---

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).

[yuanyi-350]

I ran a profiler comparison for the changed declarations in this PR.

Results (seconds):

• ContinuousLinearMap.ring_lmap_equiv_selfₗ: 0.210 -> 0.089 (-57.9%)
• ContinuousLinearMap.ring_lmap_equiv_self: 0.254 -> 0.155 (-39.0%)

Overall:

• Both changed declarations elaborated faster.

[yuanyi-350]

@grunweg , I think the current PR now meets your guidelines.

[themathqueen]

Since this is just .symm toSpanSingletonLE, we should probably deprecate it? toSpanSingletonLE has a better name anyway.

[themathqueen]

And this should arguably be called toSpanSingletonLIE or toSpanSingletonₗᵢ and flipped (i.e., E ≃ₗᵢ[𝕜] ..)? I'm not sure about the name.

[themathqueen]

There's also 0 api for these two definitions. I know this wasn't the initial plan for this PR, but can you add some basic supporting lemmas for the second one please (like _apply, _symm_apply, toLinearEquiv_, etc)?