Implemented TruffleRLanguage.toString(RContext context, Object value) and isVisible() to work properly in GraalVM shell and Interop.

Merge pull request #616 in G/fastr from ~ZBYNEK.SLAJCHRT_ORACLE.COM/fastr:extBuiltinsCasts to master

* commit '676b72bb':
  The names attribute propagated to empty vectors
  Empty vector handling fixed in the cumulative builtins
  Copyright and other minor fixes
  Cast pipelines added to external builtins. Some tests added too.

* commit '8597e4c8':
  generated expected output messages
  .colonAscending(int, double) and .colonDescending(int, double) should return RIntSequence or RDoubleSequence depending on 'right' parameter being in integer range or not.

Merge pull request #617 in G/fastr from ~LUKAS.STADLER_ORACLE.COM/fastr:bugfix/remove_arg_comments to master

* commit 'c92b30a2':
  remove unnecesarry end-of-line comments, remove unnecessary newlines

return RIntSequence or RDoubleSequence depending on 'right' parameter
being in integer range or not.

* commit '08aac059':
  generated expected output messages
  overflow problem in sum builtin
  added Output.IgnoreCase TestTrait

Merge pull request #613 in G/fastr from ~STEPAN.SINDELAR_ORACLE.COM/fastr:feature/bindist-unlist to master

* commit '369f8e22':
  unlist supports pairlists
  Stats: port external C_BinDist

* commit '431bc2c8':
  doc: refine testing.md
  add documentaion on unit/package testing

Merge pull request #607 in G/fastr from ~MICK.JORDAN_ORACLE.COM/fastr:bugfix/revertnativeproject to master

* commit 'fb93cb20':
  add back Eclipse .project files for native projects

* commit '9af956c5':
  Update copyright year in file headers after merge
  Add build support for Linux/SPARC

* commit 'ad8a935d':
  switch to flat DSL layout; more guards as Nodes
  rewrite seq.int coalesce all seq builtins into SeqFunctions add fast paths for seq and seq.default refactor and expand seq unit tests

* commit 'ee319733':
  TruffleBoundary fixes

* commit '3306ce41':
  RAttributeProfiles removed
  Copyright years fixed
  the dimensions accessors reviewed
  The dimnames and names accessor methods made default in RAbstractContainer
  Getting rid of RAttributeProfiles from RAttributable

Merge pull request #604 in G/fastr from ~MILOSLAV.METELKA_ORACLE.COM/fastr:unexpected_symbol_in_test_outputs to master

* commit '8d482e23':
  Fixing tests that unintentionally fail with Unexpected symbol error.

Merge pull request #602 in G/fastr from ~LUKAS.STADLER_ORACLE.COM/fastr:feature/sync_promise to master

* commit '9b55ddc9':
  refactor RPromise: change state to bitfield, synchronized evaluation of explicit promises, fast path for eager promises in MissingNode

* commit 'a6a696a5':
  added currently failing test in grep
  fix copyright year
  generated expected output messages
  handle multi-byte characters in SubAdapter
  java.lang.String.split(separator) does not correspond to R strsplit(..., perl=FALSE)