Properly collect functions for nested modules

[randycoulman]

Fixes #40

Thanks for this library! I've been looking for something like this for a while, and I'm glad I stumbled across it.

There were two issues:

• The parent of a nested module would include all of the functions from its nested modules

• A nested module wouldn't include any of its functions

The first issue occurred because ModuleInformation.load_user_defined_functions wasn't taking into account nested defmodule statements.

To fix this:

• Add a nesting_level to the accumulator when looking for functions.
• Switch to using Macro.traverse/4 so that we can properly keep track of the current module nesting level.
• Ignore any def statements at nesting level > 1.

The second issue was caused because the modules map in Module.load_user_defined_functions was not using fully-qualified names for the nested modules. As a result, when attempting to find the nested module's functions (which uses the fully-qualified name), the module's AST wasn't found.

To fix this:

• Convert the accumulator to include a module stack.
• Switch to using Macro.traverse/4 so that we can properly maintain the stack as we enter and exit modules.
• Compute the fully-qualified name of any nested module and use that as a key in the modules map.

Note that nested modules can also include defimpls, so we also check for those when looking for functions.

This was the best way I could find to fix the issues with nested modules, but I'm happy to consider better solutions now that I understand the root of the problem.

I ran this version against our codebase and it fixes all of the false positives we had that were caused by this bug.

[akoutmos]

This is really awesome work. Thanks!