Proposal: add complex ln_1p()

[Expander]

I'd propose to add a complex equivalent of the real function ln_1p().

This function appears in several applications that involve complex analysis.

[cuviper]

Sure, I'm open to it. Maybe exp_m1 as well?

[soumyasen1809]

Hi @cuviper I am new to Rust and would like to give this item a shot. Is this available to work on?

[cuviper]

Sure, @soumyasen1809, go ahead!

[soumyasen1809]

@cuviper Hi, I opened a PR for this
#131