Unit tests for cast pipelines builder + small fixes to have consistent semantics

...in/src/com/oracle/truffle/r/nodes/builtin/base/Array.java

...c/com/oracle/truffle/r/nodes/builtin/base/MakeUnique.java

...in/src/com/oracle/truffle/r/nodes/builtin/base/Merge.java

...n/src/com/oracle/truffle/r/nodes/builtin/base/Repeat.java

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

...c/com/oracle/truffle/r/nodes/builtin/CastBuilderTest.java

...s/src/com/oracle/truffle/r/nodes/builtin/CastBuilder.java

.../src/com/oracle/truffle/r/nodes/builtin/RBuiltinNode.java

...com/oracle/truffle/r/nodes/builtin/casts/MessageData.java

...om/oracle/truffle/r/nodes/builtin/casts/PipelineStep.java

...cle/truffle/r/nodes/builtin/casts/PipelineToCastNode.java

...fle/r/nodes/builtin/casts/fluent/InitialPhaseBuilder.java

...truffle/r/nodes/builtin/casts/fluent/PipelineBuilder.java

...e/r/nodes/builtin/casts/fluent/PipelineConfigBuilder.java

...odes/src/com/oracle/truffle/r/nodes/unary/BypassNode.java

.../com/oracle/truffle/r/nodes/unary/CastDoubleBaseNode.java

....nodes/src/com/oracle/truffle/r/nodes/unary/CastNode.java

.../com/oracle/truffle/r/nodes/unary/CastStringBaseNode.java

.../src/com/oracle/truffle/r/nodes/unary/CastStringNode.java