rlang 0.3.1

This patch release polishes the new backtrace feature introduced in
rlang 0.3.0 and solves bugs for the upcoming release of purrr
0.3.0. It also features as_label() and as_name() which are meant
to replace quo_name() in the future. Finally, a bunch of deparsing
issues have been fixed.

Backtrace fixes

• New entrace() condition handler. Add this to your RProfile to
  enable rlang backtraces for all errors, including warnings promoted
  to errors:

  if (requireNamespace("rlang", quietly = TRUE)) {
    options(error = rlang::entrace)
  }

  This handler also works as a calling handler:

  with_handlers(
    error = calling(entrace),
    foo(bar)
  )

  However it's often more practical to use with_abort() in that case:

  with_abort(foo(bar))

• with_abort() gains a classes argument to promote any kind of
  condition to an rlang error.

• New last_trace() shortcut to print the backtrace stored in the
  last_error().

• Backtrace objects now print in full by default.

• Calls in backtraces are now numbered according to their position in
  the call tree. The numbering is non-contiguous for simplified
  backtraces because of omitted call frames.

• catch_cnd() gains a classes argument to specify which classes of
  condition to catch. It returns NULL if the expected condition
  could not be caught (#696).

as_label() and as_name()

The new as_label() and as_name() functions should be used instead
of quo_name() to transform objects and quoted expressions to a
string. We have noticed that tidy eval users often use quo_name() to
extract names from quosured symbols. This is not a good use for that
function because the way quo_name() creates a string is not a well
defined operation.

For this reason, we are replacing quo_name() with two new functions
that have more clearly defined purposes, and hopefully better names
reflecting those purposes. Use as_label() to transform any object to
a short human-readable description, and as_name() to extract names
from (possibly quosured) symbols.

Create labels with as_label() to:

• Display an object in a concise way, for example to labellise axes
  in a graphical plot.

• Give default names to columns in a data frame. In this case,
  labelling is the first step before name repair.

We expect as_label() to gain additional parameters in the future,
for example to control the maximum width of a label. The way an object
is labelled is thus subject to change.

On the other hand, as_name() transforms symbols back to a string in
a well defined manner. Unlike as_label(), as_name() guarantees the
roundtrip symbol -> string -> symbol.

In general, if you don't know for sure what kind of object you're
dealing with (a call, a symbol, an unquoted constant), use
as_label() and make no assumption about the resulting string. If you
know you have a symbol and need the name of the object it refers to,
use as_name(). For instance, use as_label() with objects captured
with enquo() and as_name() with symbols captured with ensym().

Note that quo_name() will only be soft-deprecated at the next major
version of rlang (0.4.0). At this point, it will start issuing
once-per-session warnings in scripts, but not in packages. It will
then be deprecated in yet another major version, at which point it
will issue once-per-session warnings in packages as well. You thus
have plenty of time to change your code.

Minor fixes and features

• New is_interactive() function. It serves the same purpose as
  base::interactive() but also checks if knitr is in progress and
  provides an escape hatch. Use with_interactive() and
  scoped_interactive() to override the return value of
  is_interactive(). This is useful in unit tests or to manually turn
  on interactive features in RMarkdown outputs

• calling() now boxes its argument.

• New done() function to box a value. Done boxes are sentinels to
  indicate early termination of a loop or computation. For instance,
  it will be used in the purrr package to allow users to shortcircuit
  a reduction or accumulation.

• new_box() now accepts additional attributes passed to structure().

• Fixed a quotation bug with binary operators of zero or one argument
  such as `/`(1) (#652). They are now deparsed and printed
  properly as well.

• New call_ns() function to retrieve the namespace of a
  call. Returns NULL if the call is not namespaced.

• Top-level S3 objects are now deparsed properly.

• Empty { blocks are now deparsed on the same line.

• Fixed a deparsing issue with symbols containing non-ASCII
  characters (#691).

• expr_print() now handles [ and [[ operators correctly, and
  deparses non-syntactic symbols with backticks.

• call_modify() now respects ordering of unnamed inputs. Before this
  fix, it would move all unnamed inputs after named ones.

• as_closure() wrappers now call primitives with positional
  arguments to avoid edge case issues of argument matching.

• as_closure() wrappers now dispatch properly on methods defined in
  the global environment (tidyverse/purrr#459).

• as_closure() now supports both base-style (e1 and e2) and
  purrr-style (.x and .y) arguments with binary primitives.

• exec() takes .fn as first argument instead of f, for
  consistency with other rlang functions.

• Fixed infinite loop with quosures created inside a data mask.

• Base errors set as parent of rlang errors are now printed
  correctly.