[GR-2767] Channel connections.

.../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/ChannelConnections.java

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