rlang 0.1.1

Tidy evaluation fixes

This release includes two important fixes for tidy evaluation:

• Bare formulas are now evaluated in the correct environment in
  tidyeval functions.

• enquo() now works properly within compiled functions. Before this
  release, constants optimised by the bytecode compiler couldn't be
  enquoted.

New functions:

• The new_environment() constructor creates a child of the empty
  environment and takes an optional named list of data to populate it.
  Compared to env() and child_env(), it is meant to create
  environments as data structures rather than as part of a scope
  hierarchy.

• The new_language() constructor creates calls out of a callable
  object (a function or an expression) and a pairlist of arguments. It
  is useful to avoid costly internal coercions between lists and
  pairlists of arguments.

UI improvements:

• env_child()'s first argument is now .parent instead of parent.

• mut_ setters like mut_attrs() and environment helpers like
  env_bind() and env_unbind() now return their (modified) input
  invisibly. This follows the tidyverse convention that functions
  called primarily for their side effects should return their input
  invisibly.

• is_pairlist() now returns TRUE for NULL. We added is_node()
  to test for actual pairlist nodes. In other words, is_pairlist()
  tests for the data structure while is_node() tests for the type.

Bugfixes:

• env() and env_child() can now get arguments whose names start
  with .. Prior to this fix, these arguments were partial-matching
  on env_bind()'s .env argument.

• The internal replace_na() symbol was renamed to avoid a collision
  with an exported function in tidyverse. This solves an issue
  occurring in old versions of R prior to 3.3.2 (#133).