Add unnecessary apply rule

[arjank]

This PR adds a rule to check for unnecessary use of apply, as described in issue #1214.

[Mauin]

How about a test case where apply is called with unnecessary parentheses a.apply({ test = true }).

[arjank]

That should be covered by the existing UnnecessaryParentheses check.

[Mauin]

That's true. But not only that, it would also be an unnecessary apply. If we add this test we ensure that the user doesn't run detekt, realizes the UnnecessaryParentheses, fixes it, runs detekt again only to see yet another issue on the exact line of code.
If we ensure both rules report at the same time it gives the report more information that the user can act on.

[arjank]

Makes sense. I've added some more tests, including one to cover this scenario.

[Mauin]

Thank you for the contribution!

[egor-n]

Would also be nice to add a test case where the return value of apply is used:

fun a(i: Int) {
}

a(1.apply {
  println(this)
})

[arjank]

Would also be nice to add a test case where the return value of apply is used:

fun a(i: Int) {
}

a(1.apply {
  println(this)
})

I've added some tests for this.

[schalkms]

Thanks for implementing this. I like this rule.

[schalkms]

Ca we please use separate test cases for each code sample. This makes it easier to debug and trace it.

[schalkms]

The same applies here.

[Mauin]

Thank you for the additional tests! As @schalkms mentioned it makes sense splitting them into individual test cases to make them easier to maintain and more self-explanatory.

[arturbosch]

Thanks for the contribution!