Pretty-print Lambda (issue 2401)

TODO: rebase
Print Lambda using the `\mapsto` equivalent \u21A6: ↦. It looks rather small in a fixed-width font, but I didn't find any reasonable way of simulating it in Unicode-art.

This fixes issue 2401 and is an alternate proposal for #388.

Here's what it looks like:

```
In [1]: Lambda((x, y), x*y)+1
Out[1]: 1 + ((x, y) ↦ x⋅y)

In [2]: Lambda(x, x**2)**2
Out[2]:
        2
⎛     2⎞
⎝x ↦ x ⎠

In [3]: RootSum(x**5 + 11*x - 2, Lambda(z, exp(z)))
Out[3]:
       ⎛ 5                  z⎞
RootSum⎝x  + 11⋅x - 2, z ↦ ℯ ⎠
```