Plugin expansion to add factory method structure checks.

[JakeWharton]

Expand the buildSrc plugin to verify certain commonalities to these factory methods:

• @CheckResult on method
• @NonNull on method (Normalize @NonNull / checkNotNull usage #20)
• @NonNull on all parameters (Normalize @NonNull / checkNotNull usage #20)
• Action1 return types use a wildcard bound (? super Integer)
• Second parameters which are a Func1 use a wildcard for first parameterized type (? super ViewEvent)

[ZacSweers]

Part of the way through implementing this, and just realized I should probably ask: Would this be part of the kotlin generation process or separate plugin functionality? I think I know the answer, but just to be clear...

Option 1 - Kotlin Gen

Pros:

• Easy to plug in to the current code, no refactoring for sources retrieval code necessary.

Cons:

• Would only run on kotlin generation

Option 2 - Separate plugin implementation

Pros:

• Focused purpose and separation makes it easy to iterate on these checks for the future without tying them to kotlin generation
• Depending on how fast it is, could add this as a preBuild task for more aggressive checking. Disablable via "noverify" param or something.

Cons:

• None, really. Just more work, but probably for the better.

[ZacSweers]

I liked option 2, and went with that if you want to take a look before I PR: https://github.com/hzsweers/RxBinding/tree/z/structured_checks

[JakeWharton]

Option 2 is fine, although sharing code between the two plugins shouldn't be out of the question. I don't think it needs the option to disable, create a task called validateBindings (or something) and make the check task on the project depend on it.

[JakeWharton]

Looks like it already caught some missing things, too!