Small optimization on "deserialization" and fix on benchmarks again (#7982)

* [Kotlin] Small optimizations and benchmark on deserialization

* [Kotlin] Remove redudant assign() method (use init() instead)

* [Kotlin] Fix benchmark run after change in flatbuffers-java deps

Commit 6e214c3a49 fixes Kotlin build,
but makes the kotlin-benchmark plugin misses the java classes at
runtime, causing NotClassFoundError. The alternative to solve the issue
is to read java's pom.xml to get the latest java version and use it
as dependency. With that we avoid compilation errors on a new version and
keep benchmark plugin happy.

kotlin/benchmark/build.gradle.kts

@@ -1,3 +1,5 @@
+import groovy.xml.XmlParser
+
 plugins {
   kotlin("multiplatform")
   id("org.jetbrains.kotlinx.benchmark")
@@ -8,6 +10,18 @@ plugins {
 group = "com.google.flatbuffers.jmh"
 version = "2.0.0-SNAPSHOT"
 
+// Reads latest version from Java's runtime pom.xml,
+// so we can use it for benchmarking against Kotlin's
+// runtime
+fun readJavaFlatBufferVersion(): String {
+  val pom = XmlParser().parse(File("../java/pom.xml"))
+  val versionTag = pom.children().find {
+    val node = it as groovy.util.Node
+    node.name().toString().contains("version")
+  } as groovy.util.Node
+  return versionTag.value().toString()
+}
+
 // This plugin generates a static html page with the aggregation
 // of all benchmarks ran. very useful visualization tool.
 jmhReport {
@@ -55,13 +69,13 @@ kotlin {
         implementation(kotlin("stdlib-common"))
         implementation(project(":flatbuffers-kotlin"))
         implementation(libs.kotlinx.benchmark.runtime)
+        implementation("com.google.flatbuffers:flatbuffers-java:${readJavaFlatBufferVersion()}")
         // json serializers
         implementation(libs.moshi.kotlin)
         implementation(libs.gson)
       }
       kotlin.srcDir("src/jvmMain/generated/kotlin/")
       kotlin.srcDir("src/jvmMain/generated/java/")
-      kotlin.srcDir("../../java/src/main/java")
     }
   }
 }