Fix support for classes w/ multiple constructors in code gen (#976)

* Fix broken test

This test suite doesn't run on CI builds but fails locally since the method was moved

* Add multiple constructors test case

* Implement TypeName.asTypeBlock()

* Make DEFAULT_CONSTRUCTOR_MARKER public

* Look up constructor via getDeclaredConstructor with exact param types

Resolves #975

* Remove dead code

kotlin/reflect/src/main/test/java/com/squareup/moshi/kotlin/reflect/KotlinJsonAdapterTest.kt

@@ -15,7 +15,7 @@ class KotlinJsonAdapterTest {
     val moshi = Moshi.Builder()
         .add(KotlinJsonAdapterFactory())
         .build()
-    val adapter = moshi.adapter<Data>()
+    val adapter = moshi.adapter(Data::class.java)
     assertThat(adapter.toString()).isEqualTo(
         "KotlinJsonAdapter(com.squareup.moshi.kotlin.reflect.KotlinJsonAdapterTest.Data).nullSafe()"
     )