Improve the error analysis for the implementation of Dobinski's formula to compute the Bell numbers leading to

1. code which is actually consistent with the mathematical formulas
2. code which is cleaner and simpler than before
3. a complete error estimate including the final real approximation
4. faster code:

Before:

sage: timeit('bell_number(200)', number=2000)
2000 loops, best of 3: 1.45 ms per loop

After:

sage: timeit('bell_number(200)', number=2000)
2000 loops, best of 3: 441 µs per loop

Depends on #16428
Depends on #15300

Component: combinatorics

Author: Jeroen Demeyer

Branch/Commit: 330617c

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/17157