Update to 0.9

Upstream changes:
Version 0.9
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* Added an argument `allow_error` to `parse_all()` to allow syntactical errors
  in R source code when `allow_error = TRUE`; this means `evaluate(stop_on_error
  = 0 or 1)` will no longer stop on syntactical errors but returns a list of
  source code and the error object instead. This can be useful to show
  syntactical errors for pedagogical purposes.

(wen)