Generate optimized toString() (#957)

* Generate optimized toString()

This optimizes `toString()` functions to emit separate strings for the enclosing prefix and the adapter name

Example:

```kotlin
override fun toString(): String = buildString {
    append("GeneratedJsonAdapter(").append("SmokeTestType").append(")") }
```

* Optimize with capacity and char

kotlin/codegen/src/main/java/com/squareup/moshi/kotlin/codegen/api/AdapterGenerator.kt

@@ -21,6 +21,7 @@ import com.squareup.kotlinpoet.CodeBlock
 import com.squareup.kotlinpoet.FileSpec
 import com.squareup.kotlinpoet.FunSpec
 import com.squareup.kotlinpoet.KModifier
+import com.squareup.kotlinpoet.MemberName
 import com.squareup.kotlinpoet.NameAllocator
 import com.squareup.kotlinpoet.ParameterSpec
 import com.squareup.kotlinpoet.ParameterizedTypeName
@@ -39,6 +40,8 @@ import java.lang.reflect.Constructor
 import java.lang.reflect.Type
 
 private val MOSHI_UTIL = Util::class.asClassName()
+private const val TO_STRING_PREFIX = "GeneratedJsonAdapter("
+private const val TO_STRING_SIZE_BASE = TO_STRING_PREFIX.length + 1 // 1 is the closing paren
 
 /** Generates a JSON adapter for a target type. */
 internal class AdapterGenerator(
@@ -155,11 +158,19 @@ internal class AdapterGenerator(
   }
 
   private fun generateToStringFun(): FunSpec {
+    val name = originalTypeName.rawType().simpleNames.joinToString(".")
+    val size = TO_STRING_SIZE_BASE + name.length
     return FunSpec.builder("toString")
         .addModifiers(KModifier.OVERRIDE)
         .returns(String::class)
-        .addStatement("return %S",
+        .addStatement(
-            "GeneratedJsonAdapter(${originalTypeName.rawType().simpleNames.joinToString(".")})")
+            "return %M(%L)·{ append(%S).append(%S).append('%L') }",
+            MemberName("kotlin.text", "buildString"),
+            size,
+            TO_STRING_PREFIX,
+            name,
+            ")"
+        )
         .build()
   }