Add inference when using dependency injection via module attribute

[gugahoa]

Implements elixir-lsp/elixir-ls#541

This PR adds basic inference when doing dependency injection using module attributes that calls into Application.compile_env/3, Application.compile_env!/2, Application.get_env/3``and Application.fetch_env!/2`. It works by calling the function and returning the resulting module as the expression type

[lukaszsamson]

Nice @gugahoa, it looks promising. It shouldn't be to difficult to add support for other types as well.

but compile_env is a macro

Oh, did't know that. We can just call fetch/get instead as this is only semantic indicator that it shouldn't be called in runtime.

There's one thing that has been on my mind. I felt that the language server got slower, but I'm not sure if it's a feeling or if it's real. I'll set up a benchmark to see if there's a noticeable difference. If there's, what do you think of moving the expansion to the metadata_builder.ex instead of binding.ex?

I'll do some testing but IMO this feature should not impact other usecases. Regarding the moving, the idea was to keep MetadataBuilder rather simple. Anyway, I wouldn't try it in this PR.

[gugahoa]

We can just call fetch/get instead as this is only semantic indicator that it shouldn't be called in runtime.

Didn't think of that! It worked nicely 8b74fea

[gugahoa]

I believe it's ready for review. Do you want me to squash the fixups and mark as ready for review or do you want me to wait a bit?

Edit: There's another way of doing dependency injection that would be nice to add inference to:
@some_module Application.get_env(:app, __MODULE__, []) |> Keyword.get(:module, DefaultModule)

But to add support to that we would need to add support to parsing lists, and then add support to calling Keyword.get so I think it should be done in another PR, to keep this one small. What do you think?

[lukaszsamson]

I think it's ready. @gugahoa do you plan to do any more changes?

There's another way of doing dependency injection that would be nice to add inference to

We could handle it but I'm not convinced we should support every expression possible. So far Binding module cannot handle lists (maps, structs, tuples, atoms and integers).

[lukaszsamson]

@gugahoa Please look into test failures

[gugahoa]

@lukaszsamson fixed the tests, now the PR is ready for review

[lukaszsamson]

Thanks @gugahoa