feat(Data/Finset/Sym): prove two lemmas

[AntoineChambert-Loir]

Prove:

• Finset.sym_map, a rewriting lemma for Sym.map
• Sym.val_prod_eq_prod_count_pow, evaluate the product of the elements in a Sym

co-authored with @mariainesdff

---

[github-actions]

PR summary d21cadecda

Import changes for modified files

Dependency changes

File	Base Count	Head Count	Change
Mathlib.Data.Finset.Sym	532	537	+5 (+0.94%)

Import changes for all files

Files	Import difference
Mathlib.Data.Finset.Sym Mathlib.Data.Sym.Sym2.Finsupp	5

---

Declarations diff

+ sym_map
+ val_prod_eq_prod_count_pow

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).