[suggestion] Pass subject as parameter to Assertion.Builder callbacks

When writing custom assertions, the only way to get the subject is directly from the Assertion receiver’s subject parameter. A more idiomatic, Kotlin-y way to access the subject would be having it passed as the sole argument to the callback, which can be accessed implicitly as the it parameter, or be named as needed by the end-user.

The new signature would look something like:

// in strikt/api/Assertion.kt
fun assert(
      description: String,
      expected: Any?,
      assert: AtomicAssertion<T>.(T) -> Unit
    ): Builder<T>

Example usage:

fun Assertion.Builder<LocalDate>.isStTibsDay() =
  assert("is St. Tib's Day") {
    when (MonthDay.from(it)) {
      MonthDay.of(2, 29) -> pass()
      else               -> fail()
    }
  }

or

fun Assertion.Builder<LocalDate>.isStTibsDay() =
  assert("is St. Tib's Day") { dateUnderTest ->
    when (MonthDay.from(dateUnderTest)) {
      MonthDay.of(2, 29) -> pass()
      else               -> fail()
    }
  }

[robfletcher]

Implemented this on a branch. See f28f3f9. I think it simplifies some things e.g.

• Assertion and derived types no longer require a generic type.
• was able to eliminate the AssertionComposer interface altogether.

[robfletcher]

Implemented this. It affects assert, compose, map and passesIf.