Introduce @DeprecatedVisitor; mark up visitor-related declarations with it.

Provide convenient JVM writing API.

Get rid of ExperimentalUnsignedTypes.

plugins/jvm-abi-gen/src/org/jetbrains/kotlin/jvm/abi/JvmAbiMetadataProcessor.kt

@@ -18,6 +18,7 @@ import org.jetbrains.org.objectweb.asm.Opcodes
  * Wrap the visitor for a Kotlin Metadata annotation to strip out private and local
  * functions, properties, and type aliases as well as local delegated properties.
  */
+@OptIn(DeprecatedVisitor::class)
 fun abiMetadataProcessor(annotationVisitor: AnnotationVisitor): AnnotationVisitor =
     kotlinClassHeaderVisitor { header ->
         // kotlinx-metadata only supports writing Kotlin metadata of version >= 1.4, so we need to
@@ -136,6 +137,7 @@ private fun AnnotationVisitor.visitKotlinMetadata(header: KotlinClassHeader) {
  * Class metadata adapter which removes private functions, properties, type aliases,
  * and local delegated properties.
  */
+@DeprecatedVisitor
 private class AbiKmClassVisitor(delegate: KmClassVisitor) : KmClassVisitor(delegate) {
     override fun visitConstructor(flags: Flags): KmConstructorVisitor? {
         if (!isPrivateDeclaration(flags))
@@ -172,6 +174,7 @@ private class AbiKmClassVisitor(delegate: KmClassVisitor) : KmClassVisitor(deleg
  * Class metadata adapter which removes private functions, properties, type aliases,
  * and local delegated properties.
  */
+@DeprecatedVisitor
 private class AbiKmPackageVisitor(delegate: KmPackageVisitor) : KmPackageVisitor(delegate) {
     override fun visitFunction(flags: Flags, name: String): KmFunctionVisitor? {
         if (!isPrivateDeclaration(flags))