Implement rgamma + unified tests for all rxxx functions

...library/src/com/oracle/truffle/r/library/stats/RBeta.java

...brary/src/com/oracle/truffle/r/library/stats/RCauchy.java

...ibrary/src/com/oracle/truffle/r/library/stats/RGamma.java

...acle/truffle/r/library/stats/RandGenerationFunctions.java

...ibrary/src/com/oracle/truffle/r/library/stats/Rbinom.java

...library/src/com/oracle/truffle/r/library/stats/Rnorm.java

...library/src/com/oracle/truffle/r/library/stats/Runif.java

....library/src/com/oracle/truffle/r/library/stats/SExp.java

...library/src/com/oracle/truffle/r/library/stats/SNorm.java

...brary/src/com/oracle/truffle/r/library/stats/TOMS708.java

...ruffle/r/nodes/builtin/base/foreign/ForeignFunctions.java

....r.runtime/src/com/oracle/truffle/r/runtime/rng/RRNG.java

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

...le/truffle/r/test/library/stats/TestExternal_rcauchy.java

...acle/truffle/r/test/library/stats/TestExternal_rnorm.java

...acle/truffle/r/test/library/stats/TestExternal_runif.java

...fle/r/test/library/stats/TestRandGenerationFunctions.java

mx.fastr/copyrights/overrides