2401: Fix pretty-printing of Lambda.

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 ⎠
Merge pull request #2623 from skirpichev/rlamy-print-Lambda

pretty-printing of Lambda (issue 2401)