implement as.POSIXlt, as.POSIXct, POSIXlt2Date, format.POSIXlt and strptime

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

...acle/truffle/r/nodes/builtin/base/DatePOSIXFunctions.java

...racle/truffle/r/nodes/builtin/base/R/datetime_overrides.R

...m/oracle/truffle/r/nodes/builtin/base/R/files_overrides.R

.../truffle/r/test/testrgen/TestrGenBuiltinDate2POSIXlt.java

.../truffle/r/test/testrgen/TestrGenBuiltinPOSIXlt2Date.java

...ruffle/r/test/testrgen/TestrGenBuiltinallequalPOSIXt.java

...cle/truffle/r/test/testrgen/TestrGenBuiltinasPOSIXct.java

...cle/truffle/r/test/testrgen/TestrGenBuiltinasPOSIXlt.java

...truffle/r/test/testrgen/TestrGenBuiltinformatPOSIXlt.java

...e/truffle/r/test/testrgen/TestrGenBuiltinisnaPOSIXlt.java

...e/truffle/r/test/testrgen/TestrGenBuiltinmeanPOSIXct.java

...acle/truffle/r/test/testrgen/TestrGenBuiltinstrptime.java