Fix lib issues and restructure project

[asaaki]

Sorry, I have to propose further changes, but the gen task was not completely working for me.

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Changes:

• (Structure) Move every task into its own file
• (Mix.Tasks.Nifty.Gen) Make all non-public functions explicitly private
• (Mix.Tasks.Nifty.Gen) Add some more helper functions for cleaner structure
• (Mix.Tasks.Nifty.Gen) Fix directory creation and file paths*
• (Templates: nif.ex.eex) Fix indentation level, fix load path for lib file**
• (Templates: nif.ex.eex) Use Kernel.exit/1 if NIF loading fails***
• (Templates: nif.c.eex) Fix indentation level of function blocks (6 spaces are pretty odd)
• (Templates: Makefile.eex) Fix path for lib**

*) The NIF Elixir file will be created under lib/<name>/nif.ex to meet the general structure of a project. At least it makes more sense somehow from my POV.

**) As you can see, I reduce the path from priv/lib/lib_<file> to priv/lib_<file>. I think this is totally fine IMHO. Mostly this avoids confusion or even loading issues (one level less to care about).

***) I would say it is considerably safer to let it crash instead of returning just a string here, especially in cases where the expected return type is a string, too. This could lead to hard to debug situations, therefore the exit with the given reason points to the real issue much quicker.

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I hope you like this proposal. So far this is the state where it does not fail anymore when I create the NIF stubs for a project.

[rossjones]

Thanks, I really appreciate you taking the time to clean this up. I need to work out building on Windows next, so it helps to have it in a much better state.

[asaaki]

Yeah, regarding Windows support I have to rely totally on your feedback, because I just can work on Linux and Mac OSX. But I hope these changes do not break anything. :fingercrossed:

[asaaki]

And thank you for this awesome tool, this saves a lot of time when playing around with C libraries! :bow: