Generated Kotlin code triggers compile warnings

[larryng]

This makes using Kotlin code generation mode impossible when allWarningsAsErrors is enabled. See #205. This time it's because the generated method signature doesn't match for binds factory methods.

A few requests to address this:

1. Fix or suppress the warning in generated code (similar to PR above).
2. Enable allWarningsAsErrors in Kotlin codegen test suite.
3. Add a note in https://github.com/uber/motif/wiki#code-generation-mode for Gradle/Kotlin users (looking up how to set this was a bit painful):

   kapt {
       arguments {
           arg("motif.mode", "java")
       }
   }
   

Library version:
0.3.4

Repro steps or stacktrace:
Add a binds factory method:

@Expose
fun foo(fooImpl: FooImpl): Foo

Compile warning:

w: <redacted>: The corresponding parameter in the supertype 'Objects' is named 'fooImpl'. This may cause problems when calling this function with named arguments.