A clean-up of the sample generation code

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

...src/com/oracle/truffle/r/nodes/casts/CastNodeSampler.java

....test/src/com/oracle/truffle/r/nodes/casts/CastUtils.java

...m/oracle/truffle/r/nodes/casts/PredefMappersSamplers.java

...es.test/src/com/oracle/truffle/r/nodes/casts/Samples.java

.../src/com/oracle/truffle/r/nodes/test/ChimneySweeping.java

...com/oracle/truffle/r/nodes/test/DefaultArgsExtractor.java

.../com/oracle/truffle/r/nodes/test/RBuiltinDiagnostics.java

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

mx.fastr/mx_fastr.py