seq_along dispatches to length for objects with class

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

...s/src/com/oracle/truffle/r/nodes/control/RLengthNode.java

...s/src/com/oracle/truffle/r/nodes/unary/FindFirstNode.java

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

...oracle/truffle/r/test/builtins/TestBuiltin_seq_along.java