taylor_exp3() where we keep a list of factors, sort it first (descendingly)
and the perform the addition.

This has the advantage that we don't overflow that soon (since we don't keep total x^n and n! around).
But has the drawback of costing a lot of memory. The results are much better though, since we use our
floats better. Still getting magic numbers at some point though.
Replaced the threshold for terminating the exp() algorithms with constant ETA=0.0001
Added an exp3 that keeps separate track of the mantissa for improved accuracy, and Kahan algorithm.

The former is a bit more accurate, but still starts to be quite wrong around n=100. Mostly because the
larger terms dominate everything.

The latter implements the Kahan algorithm which does something very similar: keeping track of an error
(basically the lower order parts). It also diverges a lot around n=100.