Initial implementation of Java scanner and parser

Ported from scalac 2.11.x branch SHA 9753f23f9362b25a9f481b11dd8d51187187882a

This is mostly a direct port, with few significant dotty-specific
changes needed. The two more significant changes are:

In dotty, the first constructor of a class is pulled out separately from
the other stats in the Template.

The keyword detection code (buildKeywordArray) was moved into Tokens so
that it can more cleanly be shared by the Scala and Java scanners.

Author: olhotak

src/dotty/tools/dotc/core/Flags.scala

@@ -354,6 +354,9 @@ object Flags {
   /** Symbol is a Java default method */
   final val DefaultMethod = termFlag(39, "<defaultmethod>")
 
+  /** Symbol is a Java enum */
+  final val Enum = commonFlag(40, "<enum>")
+
   // Flags following this one are not pickled
 
   /** Denotation is in train of being loaded and completed, used to catch cyclic dependencies */