refactor Cast node hierarchy

...src/com/oracle/truffle/r/nodes/builtin/base/AsVector.java

...src/com/oracle/truffle/r/nodes/unary/CastComplexNode.java

.../src/com/oracle/truffle/r/nodes/unary/CastDoubleNode.java

...src/com/oracle/truffle/r/nodes/unary/CastIntegerNode.java

...es/src/com/oracle/truffle/r/nodes/unary/CastListNode.java

...src/com/oracle/truffle/r/nodes/unary/CastLogicalNode.java

....nodes/src/com/oracle/truffle/r/nodes/unary/CastNode.java

...des/src/com/oracle/truffle/r/nodes/unary/CastRawNode.java

.../src/com/oracle/truffle/r/nodes/unary/CastStringNode.java

.../src/com/oracle/truffle/r/nodes/unary/CastSymbolNode.java

.../com/oracle/truffle/r/nodes/unary/ConvertBooleanNode.java