slope and dslope

Stream: maths

[Yury G. Kudryashov]

I'm going to move docs#slope to a separate file, start using it in analysis/calculus, and add dslope f a b defined as slope f a b if b ≠ a and deriv f a otherwise.

[Yury G. Kudryashov]

(posting here for the case if someone else was going to do something similar)

[Yury G. Kudryashov]

(I'm formalizing the removable singularity theorem)