simplify/validate all helpers for exp/log series

[skirpichev]

That includes _eval_nseries() for Add/Mul/Pow/log. See Ch.7 of the Gruntz thesis.

• all should be valid (but not necessary efficient, e.g. may require more terms from the arguments expansions) up to the zero equivalence problem
• no random hacks (expand here, cancel there)
• any limit for exp/log expressions (with possibly constants, e.g. I, arithmetic operations, compositions) should be valid
• take branch cuts properly (this will require, probably, support for floor/arg series)
• simplify helper signature (no logx)? See Gruntz thesis, 7.3

We can include hypothesis-based bulk tests and also test results with https://github.com/WolframResearch/WolframClientForPython.

• Add._eval_nseries()
• Mul._eval_nseries()
• Pow._eval_nseries()
• log._eval_nseries()

[skirpichev]

#589