Add ln_1p and exp_m1

[soumyasen1809]

Fixes issue #123

[Expander]

I propose to add a test for ln_1p() that checks that $\ln(1+z)=z$ for $z=\epsilon+i\epsilon$, where $\epsilon=$std::f64::EPSILON. In my opinion this test should pass within 1ulp.

[Expander]

julia has a reference implementation that can be found here: https://github.com/JuliaLang/julia/blob/5ebc5b463ead44e98a3108a477084bdd591e3d8e/base/complex.jl#L748