feat(analysis/calculus): let simp compute derivatives (#2422)
After this PR:
```lean
example (x : ℝ) : deriv (λ x, cos (sin x) * exp x) x = (cos(sin(x))-sin(sin(x))*cos(x))*exp(x) :=
by { simp, ring }
```
Excerpts from the docstrings:
The simplifier is set up to prove automatically that some functions are differentiable, or differentiable at a point (but not differentiable on a set or within a set at a point, as checking automatically that the good domains are mapped one to the other when using composition is not something the simplifier can easily do). This means that one can write
`example (x : ℝ) : differentiable ℝ (λ x, sin (exp (3 + x^2)) - 5 * cos x) := by simp`.
If there are divisions, one needs to supply to the simplifier proofs that the denominators do not vanish, as in
```lean
example (x : ℝ) (h : 1 + sin x ≠ 0) : differentiable_at ℝ (λ x, exp x / (1 + sin x)) x :=
by simp [h]
```
The simplifier is not set up to compute the Fréchet derivative of maps (as these are in general complicated multidimensional linear maps), but it will compute one-dimensional derivatives.
To make sure that the simplifier can prove automatically that functions are differentiable, we tag many lemmas with the `simp` attribute, for instance those saying that the sum of differentiable functions is differentiable, as well as their product, their cartesian product, and so on. A notable exception is the chain rule: we do not mark as a simp lemma the fact that, if `f` and `g` are differentiable, then their composition also is: `simp` would always be able to match this lemma, by taking `f` or `g` to be the identity. Instead, for every reasonable function (say, `exp`), we add a lemma that if `f` is differentiable then so is `(λ x, exp (f x))`. This means adding some boilerplate lemmas, but these can also be useful in their own right.
Tests for this ability of the simplifier (with more examples) are provided in `tests/differentiable.lean`.
