Rewrite ProgressionUtil to Kotlin, move to "kotlin.internal"

Use reflection in ProgressionUtilTest instead of calling methods on the class
from the class path, because in the latter case we were just testing
ProgressionUtil from bootstrap runtime (which, presumably, always works
correctly since it's a bootstrap distribution)

With this change, compilable built-ins (under core/builtins/src) are fully
written in Kotlin

build.xml

@@ -316,7 +316,11 @@
                 <include name="**/*.class" if="${bootstrap.build.no.tests}"/>
               </zipfileset>
               <fileset dir="${output}/classes/compiler"/>
-              <fileset dir="${output}/builtins" includes="jet/**"/>
+              <fileset dir="${output}/builtins">
+                <include name="jet/**"/>
+                <include name="kotlin/**"/>
+                <exclude name="kotlin/internal/**"/>
+              </fileset>
               <fileset dir="${basedir}/compiler/frontend.java/src" includes="META-INF/services/**"/>
 
               <zipgroupfileset dir="${basedir}/lib" includes="*.jar"/>
@@ -567,7 +571,6 @@
                      classpath="${basedir}/core/builtins/src${path.separator}${basedir}/core/runtime.jvm/src"/>
 
         <javac2 destdir="${output}/classes/runtime" debug="true" debuglevel="lines,vars,source" includeAntRuntime="false" source="${java.target}" target="${java.target}">
-            <src path="${basedir}/core/builtins/src"/>
             <src path="${basedir}/core/runtime.jvm/src"/>
             <classpath location="${output}/classes/runtime"/>
         </javac2>