Implemented internal function 'truncate'.

.../com/oracle/truffle/r/nodes/builtin/base/BasePackage.java

...cle/truffle/r/nodes/builtin/base/ConnectionFunctions.java

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

.../com/oracle/truffle/r/runtime/conn/ConnectionSupport.java

...om/oracle/truffle/r/runtime/conn/DelegateRConnection.java

...racle/truffle/r/runtime/conn/DelegateReadRConnection.java

...rc/com/oracle/truffle/r/runtime/conn/FileConnections.java

...me/src/com/oracle/truffle/r/runtime/conn/RConnection.java

...src/com/oracle/truffle/r/runtime/conn/RawConnections.java

...cle/truffle/r/runtime/conn/SeekableMemoryByteChannel.java

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

...m/oracle/truffle/r/test/library/base/TestConnections.java