Add Kotlin multiplatform support (#7969)

* [Kotlin] Introduction to Kotlin Multiplaform

The first implementation of the Kotlin code generation was made years
ago at the time Kotlin Multiplaform was not stable and Kotlin is mostly
used on JVM-based targets. For this reason the generated code uses java
based runtime.

That design decision comes with many drawbacks, leaving the code
generated more java-like and making it impossible to use more advanced
features of the Kotlin language.

In this change we are adding two parts: A pure, multi-plaform, Kotlin
runtime and a new code generator to accompany it.

* [Kotlin] Remove scalar sign cast from code generation

Now that we have a new runtime the accepts unsigned types, we don't
need to code generate casting back and from signed scalars. This
MR removes this from both code generations and adds the necessary
API to the runtime.

* [Kotlin] Use offset on public API to represent buffer position

Currently, kotlin was following Java's approach of representing objects,
vectors, tables as "Int" (the position of it in the buffer). This change
replaces naked Int with Offset<T>, offering a type-safe API. So,
instead of

fun Table.createTable(b: FlatBufferBuilder, subTable: Int)

We will have

fun Table.createTable(b: FlatBufferBuilder, subTable: Offset<SubTable>)

Making impossible to accidentally switch parameters.

The performance should be similar to use Int as we are using value
class for Offset and ArrayOffset, which most of the time translate to
Int in the bytecode.

* [Kotlin] Add builder for tables

Add builder constructor to make create of table more ergonomic.
For example the movie sample for the test set could be written as:

Movie.createMovie(fbb,
    mainCharacterType = Character_.MuLan,
    mainCharacter = att) {
    charactersType = charsType
    this.characters = characters
}

instead of:

Movie.startMovie(fbb)
Movie.addMainCharacterType(fbb, Character_.MuLan)
Movie.addMainCharacter(fbb, att as Offset<Any>)
Movie.addCharactersType(fbb, charsType)
Movie.addCharacters(fbb, charsVec)
Movie.endMovie(fbb)

* [Kotlin] Move enum types to value class

Moving to flatbuffer enums to value class adds type safety for parameters
with minimum to no performance impact.

* [Kotlin] Simplify Union parameters to avoid naked casting

Just a small change on the APIs that receive union as parameters,
creating a typealias UnionOffset to avoid using Offset<Any>. To "convert"
an table offset to an union, one just call Offset.toUnion().

* [Kotlin] Apply clang-format on kotlin code generators

* [Kotlin] Update kotlin generator to follow official naming conventions

Updating directory, package and enum naming to follow Kotlin official
convention.

https://kotlinlang.org/docs/coding-conventions.html#naming-rules

* [Kotlin] Add fixes to improve performance

1 - Add benchmark comparing serialization between Java & Kotlin
2 - ReadWriteBuffer does not auto-grow (thus avoid check size in every op)
3 - Add specialized add functions on FlatBufferBuilder to avoid boxing
offsets.
4 - Remove a few Kotlin syntax sugar that generated performance penalties.

* [Kotlin] Remove builder from Kotlin KMP and add some optimizations
to avoid boxing of Offset classes

---------

Co-authored-by: Derek Bailey <derekbailey@google.com>

Author: paulovap

.github/labeler.yml

@@ -48,6 +48,7 @@ java:
 kotlin:
   - '**/*.kt'
   - src/idl_gen_kotlin.cpp
+  - src/idl_gen_kotlin_kmp.cpp
 
 lua:
   - '**/*.lua'

.github/workflows/build.yml

@@ -422,9 +422,14 @@ jobs:
       with:
         distribution: 'temurin'
         java-version: '11'
+    - name: Build flatc
+      run: |
+       cmake -DFLATBUFFERS_BUILD_TESTS=OFF -DFLATBUFFERS_BUILD_FLATLIB=OFF -DFLATBUFFERS_BUILD_FLATHASH=OFF .
+       make -j
+       echo "${PWD}" >> $GITHUB_PATH
     - name: Build
       working-directory: kotlin
-      run: ./gradlew clean iosX64Test macosX64Test
+      run: ./gradlew clean iosSimulatorArm64Test macosX64Test macosArm64Test
 
   build-kotlin-linux:
     name: Build Kotlin Linux
@@ -437,6 +442,11 @@ jobs:
         distribution: 'temurin'
         java-version: '11'
     - uses: gradle/wrapper-validation-action@v1.0.5
+    - name: Build flatc
+      run: |
+       cmake -DFLATBUFFERS_BUILD_TESTS=OFF -DFLATBUFFERS_BUILD_FLATLIB=OFF -DFLATBUFFERS_BUILD_FLATHASH=OFF .
+       make -j
+       echo "${PWD}" >> $GITHUB_PATH
     - name: Build
       working-directory: kotlin
       # we are using docker's version of gradle