Added support for vector accesses via [ and [[ functions.

...truffle/r/nodes/builtin/base/InfixEmulationFunctions.java

...tin/src/com/oracle/truffle/r/nodes/builtin/base/Scan.java

...des/access/array/PositionsArrayConversionNodeAdapter.java

...uffle/r/nodes/access/array/PositionsArrayNodeAdapter.java

...le/truffle/r/nodes/access/array/read/AccessArrayNode.java

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

...le.r.runtime/src/com/oracle/truffle/r/runtime/RError.java

...est/src/com/oracle/truffle/r/test/ExpectedTestOutput.test

...le.r.test/src/com/oracle/truffle/r/test/all/AllTests.java

...c/com/oracle/truffle/r/test/simple/TestSimpleVectors.java