Simplify object size calculation

...ry/src/com/oracle/truffle/r/library/utils/ObjectSize.java

...library/src/com/oracle/truffle/r/library/utils/Rprof.java

...rary/src/com/oracle/truffle/r/library/utils/Rprofmem.java

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

...oracle/truffle/r/runtime/data/AgentObjectSizeFactory.java

.../com/oracle/truffle/r/runtime/data/ObjectSizeFactory.java

.../truffle/r/runtime/data/OutputAgentObjectSizeFactory.java

...e/src/com/oracle/truffle/r/runtime/data/RDataFactory.java

...me/src/com/oracle/truffle/r/runtime/data/RObjectSize.java

...racle/truffle/r/runtime/data/SimpleObjectSizeFactory.java

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

...racle/truffle/r/test/builtins/TestBuiltin_objectsize.java

documentation/dev/managed_ffi.md