Lists can hold elements that are not in a consistent state with respect to the sharing model.
However, such elements are put into consistent state once they are read from the list, their
state can be inferred from the state of the owning list.

This means that temporary elements can be freely put into lists without incrementing their ref-count.
Moreover, when a list is copied into another variable or passed as an argument to a function,
we only have to increment the ref-count of the list (no recursion needed).

When accessing an element of a list, one should use ExtractListElement node, which handles
the transition to consistent state. This must be used wherever the state matters: when returning
the value as a result of a builtin, putting it into another list, putting it as an attribute, etc.
On the other hand, when one only wants to peek at the value, but then forget the reference,
it is OK to get it directly using getDataAt and similar methods.

Merge pull request #334 in G/fastr from ~LUKAS.STADLER_ORACLE.COM/fastr:performance/visibility to master

* commit 'd13af296':
  remove all non-optimized visibility schemes
  pass visibility through RCaller object

Merge pull request #332 in G/fastr from ~MICK.JORDAN_ORACLE.COM/fastr:feature/apache-compress to master

* commit '52cdbbcb':
  backout internal RCompression changes
  implement bzip2 decompression internally

Merge pull request #336 in G/fastr from ~LUKAS.STADLER_ORACLE.COM/fastr:bugfix/transfer_to_interpreter_debug to master

* commit 'f4279129':
  transferToInterpreter in DebugHandling

Merge pull request #335 in G/fastr from ~LUKAS.STADLER_ORACLE.COM/fastr:performance/intrinsify_strings to master

* commit 'd252df6f':
  intern strings during deserialization

Merge pull request #338 in G/fastr from ~LUKAS.STADLER_ORACLE.COM/fastr:bugfix/attriubte_copy_right to master

* commit '90fd7e8d':
  fix copying of attributes from right argument

Merge pull request #337 in G/fastr from ~LUKAS.STADLER_ORACLE.COM/fastr:bugfix/order_collation to master

* commit '20ca48d6':
  fix use of collation in "order"

* commit 'e813ef20':
  cleanups

* commit 'dab41646':
  TypeConvert external: tests + few fixes

* commit '7c2090e1':
  WriteTable: fix raw formatting
  WriteTable: use RConnection instead of InputStream, fix double formatting + tests

* commit '47c286f0':
  Fix slow type check for external TruffleObjects.

* commit '2bfa2f3a':
  Always use /bin/sh in system as per GNU R; get visibility correct

Merge pull request #311 in G/fastr from ~STEPAN.SINDELAR_ORACLE.COM/fastr:feature/refactoring_colSums to master

* commit '4dfa3405':
  RowSums, ColSums, RowMeans, ColMeans common code refactored including cast pipeline

Merge pull request #328 in G/fastr from ~STEPAN.SINDELAR_ORACLE.COM/fastr:fix/formulae-polishing to master

* commit 'ae43e802':
  RInternalCodeBuiltinNode does not increment ref-count for arguments: note about it in the comments + fix in R code of termsform.
  RLanguage does not override isObject
  termsform resets attributes of the result

RInternalCodeBuiltinNode does not increment ref-count for arguments: note about it in the comments + fix in R code of termsform.

Additionally: the tests against GnuR externals now check the result
of the external call as opposed to checking the result of the wrapper
of the external call. This helps to find even tinier differences.

* commit 'c15ae475':
  A fix to vector update to preserve types in case when no actual modification happens.
  Fixed a problem with handling .Environment attribute.

Merge pull request #323 in G/fastr from ~MICK.JORDAN_ORACLE.COM/fastr:feature/apache-compress to master

* commit '67ecb276':
  Use xz internal for RCompression.lzmaUncompressFromFile