feat(data/power_series): Add multivariate power series and prove basic API

[jcommelin]

formal power series

This file defines (multivariate) formal power series
and develops the basic properties of these objects.

A formal power series is to a polynomial like an infinite sum is to a finite sum.

We provide the natural inclusion from polynomials to power series.

trunc n φ truncates a power series to the polynomial
that has the same coefficients as φ, for all m ≤ n, and 0 otherwise.

If the constant coefficient of a formal power series is invertible,
then this formal power series is invertible.

Formal power series over a local ring form a local ring.

Implementation notes

In this file we define multivariate power series with coefficients in α as
mv_power_series σ α := (σ →₀ ℕ) → α
Unfortunately there is not yet enough API to show that they are the completion
of the ring of multivariate polynomials. However, we provide most of the infrastructure
that is needed to do this. Once I-adic completion (topological or algebraic) is available
it should not be hard to fill in the details.

Formal power series in one variable are defined as
power_series α := mv_power_series unit α
This allows us to port a lot of proofs and properties
from the multivariate case to the single variable case.
However, it means that power series are indexed by (unit →₀ ℕ),
which is of course canonically isomorphic to ℕ.
We then build some glue to treat power series as if they are indexed by ℕ.
Occasionally this leads to proofs that are uglier than expected.