Rename Pattern to Regex.

toRegex now converts string to our regex, and toPattern converts to JVM Pattern.

libraries/stdlib/test/text/StringUtilTest.kt

@@ -5,8 +5,8 @@ import kotlin.test.*
 import org.junit.Test as test
 
 class StringUtilTest() {
-    test fun toRegex() {
-        val re = """foo""".toRegex()
+    test fun toPattern() {
+        val re = """foo""".toPattern()
         val list = re.split("hellofoobar").toList()
         assertEquals(listOf("hello", "bar"), list)
     }