pull Engine interface into separate file

...fle.r.engine/src/com/oracle/truffle/r/engine/REngine.java

...le/truffle/r/engine/repl/debug/RDebugSupportProvider.java

...n/src/com/oracle/truffle/r/nodes/builtin/base/Lapply.java

...in/src/com/oracle/truffle/r/nodes/builtin/base/Parse.java

...nodes/src/com/oracle/truffle/r/nodes/InlineCacheNode.java

...nodes/src/com/oracle/truffle/r/nodes/control/ForNode.java

...e/src/com/oracle/truffle/r/runtime/RRuntimeASTAccess.java

....runtime/src/com/oracle/truffle/r/runtime/RSerialize.java

...time/src/com/oracle/truffle/r/runtime/context/Engine.java

...me/src/com/oracle/truffle/r/runtime/context/RContext.java