feat(Fourier): improved version of Riemann-Lebesgue

[mcdoll]

---

The Riemann-Lebesgue lemma is merely a side note of the fact that we need to relate the extension of the Schwartz Fourier transform to L1 to the original one. RL on Schwartz is trivial by embedding into the correct space.

[github-actions]

PR summary 4a1339f2f3

Import changes exceeding 2%

%	File
+9.96%	Mathlib.Analysis.Fourier.RiemannLebesgueLemma

Import changes for modified files

Dependency changes

File	Base Count	Head Count	Change
Mathlib.Analysis.Fourier.RiemannLebesgueLemma	2461	2706	+245 (+9.96%)

Import changes for all files

Files	Import difference
Mathlib.Analysis.Fourier.RiemannLebesgueLemma	245
Mathlib.Analysis.Fourier.L1Space (new file)	2702

---

Declarations diff

+ Lp.fourierTransformCLM
+ Lp.fourierTransformCLM_apply_apply
+ Lp.fourierTransformCLM_toBCF
+ Lp.fourierTransformCLM_toLp_one_apply
+ _root_.LinearMap.mkContinuous'
+ foo
+ foo_apply
+ foo₀
+ foo₀_apply
+ fooₗ
+ fooₗ_apply
+ fourierIntegral_congr_ae
+ fourier_congr_ae
+ instance : ContinuousEval C₀(α, β) α β
+ instance : ContinuousEvalConst C₀(α, β) α β := inferInstance
+ lipschitz_eval_const
+ norm_fourier_Lp_top_leq_toLp_one
+ norm_fourier_apply_le_toLp_one
+ norm_fourier_toBoundedContinuousFunction_le_toLp_one
+ norm_fourier_toZeroAtInfty_le_toLp_one
+ norm_toBoundedContinuousFunction_le
+ norm_toLp'
+ norm_toLp_one
+ norm_toLp_top_le
+ norm_toZeroAtInfty
+ riemann_lebesgue

You can run this locally as follows

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

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

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

---

No changes to technical debt.

You can run this locally as

./scripts/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).

[mathlib-merge-conflicts]

This pull request has conflicts, please merge master and resolve them.