First try at nullable `mapN` <https://github.com/...

# arrow-contributors

[tavish pegram]

First try at nullable mapN https://github.com/arrow-kt/arrow-core/pull/189 Would appreciate any feedback!

[aballano]

Thanks for the contribution! I’m reviewing it at the moment 🙂

[tavish pegram]

@aballano updated based on your comments! thanks for showing me the Gen stuff. I left in mapN with 1 argument, but can remove it if we just want to consumers to use a?.let { fn(it) }