feat(FormalGroup): APIs for formal group homomorphism

[WenrongZou]

This PR introduces some APIs for homomorphism between formal group laws.

---

• depends on: feat(FormalGroup): F(X,0)=X and F(0,X)=X #38052
• depends on: feat(PowerSeries): rename a power series as multivariate power series #37618

[github-actions]

PR summary 896cc56a39

Import changes exceeding 2%

%	File
+19.57%	Mathlib.RingTheory.FormalGroup.Basic
+11.02%	Mathlib.RingTheory.MvPowerSeries.Equiv
+27.43%	Mathlib.RingTheory.MvPowerSeries.Rename

Import changes for modified files

Dependency changes

File	Base Count	Head Count	Change
Mathlib.RingTheory.MvPowerSeries.Rename	1130	1440	+310 (+27.43%)
Mathlib.RingTheory.FormalGroup.Basic	1477	1766	+289 (+19.57%)
Mathlib.RingTheory.MvPowerSeries.Equiv	1588	1763	+175 (+11.02%)

Import changes for all files

Files	Import difference
Mathlib.RingTheory.MvPowerSeries.Equiv	175
Mathlib.RingTheory.FormalGroup.Basic	289
Mathlib.RingTheory.MvPowerSeries.Rename	310

---

Declarations diff

+ FormalGroup.assoc'
+ FormalGroupHom
+ FormalGroupHom.applyPoint
+ FormalGroupHom.applyPoint_val
+ FormalGroupHom.map_add
+ FormalGroupHom.toAddMonoidHom
+ FormalGroupHom.toAddMonoidHom_apply
+ FormalGroupIso
+ FormalGroupIso.IsStrict
+ FormalGroupIso.ext_iff'
+ FormalGroupIso.invHom_subst_toHom
+ FormalGroupIso.toHom_subst_invHom
+ HasSubst.X_comp
+ HasSubst.X_zero
+ HasSubst.toMvPowerSeries
+ HasSubst.zero_X
+ MvPowerSeries.rename_comp_toMvPowerSeries
+ MvPowerSeries.rename_toMvPowerSeries
+ X_add_zero_eq_X
+ X_apply
+ Xzero
+ Xzero_subst_Xzero
+ _root_.MvPowerSeries.monomial_mapDomain_eq_prod
+ add_zero
+ coeff_coeToMvPowerSeries
+ constantCoeff_Xzero
+ constantCoeff_zeroX
+ instance : AddZeroClass (F.Point σ)
+ instance : CoeFun (FormalGroupHom F G) (fun _ ↦ {σ : Type*} → F.Point σ → G.Point σ)
+ instance : Zero (F.Point σ)
+ isNilpotent_constCoeff_subst_of_isNilpotent_constCoeff
+ rename_eq_subst
+ subst_eq_id_iff_eq_X
+ toMvPowerSeries
+ toMvPowerSeries_C
+ toMvPowerSeries_X
+ toMvPowerSeries_apply
+ toMvPowerSeries_eq_subst
+ toMvPowerSeries_injective
+ toMvPowerSeries_val
+ zeroX
+ zeroX_subst_zeroX
+ zero_add
+ zero_add_X_eq_X
++ subst_zero_of_constantCoeff_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.

---

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).

[mathlib-dependent-issues]

This PR/issue depends on:

• feat(FormalGroup): F(X,0)=X and F(0,X)=X #38052
• feat(PowerSeries): rename a power series as multivariate power series #37618
  By Dependent Issues (🤖). Happy coding!