Support for inline classes

[TjeuKayim]

This pull request implements support for Kotlin's new inline classes.

Solves issue #199.

Jackson can't deserialize an inline class, because the constructor is private in the generated bytecode.
However, the synthetic method box-impl can be used as JsonCreator. So I added some code to KotlinNamesAnnotationIntrospector.hasCreatorAnnotation().

Previously, inline classes where serialized as object, i.e. {"value":123}. But it should just serialize as a number: 123.
So the property needs to be annotated with @JsonValue. It would be useful to do this by default for inline classes. That's why I overrided hasAsValue().

With these two changes, inline classes can be read and written to JSON without any configuration.

inline class Watt(val value: Long)

@Test
fun readInlineClass() {
    val watt = Watt(1234)
    val json = "1234"
    val result = mapper.readValue<Watt>(json)
    assertEquals(watt, result)
}

@Test
fun writeInlineClass() {
    val watt = Watt(1234)
    val json = mapper.writeValueAsString(watt)
    assertEquals("1234", json)
}

[TjeuKayim]

I found a problem with my current implementation. Classes that contain a property with inline class type, can't be constructed. It’s because in the bytecode the field is unboxed.

inline class IC(val u: Int)
class HasInlineClassProperty(val ic: IC)

So we also need to cover cases like this.

[TjeuKayim]

Sorry, I need to stop working on this feature, because it’s too much work.

My current progress:
https://github.com/TjeuKayim/jackson-module-kotlin/commits/class-descriptor-reflection

I hope my contribution will be useful.
Good luck finishing the support for inline classes!