Improve `rust_source()` and related functions

[Ilia-Kosenkov]

This topic tracks a number of small issues I found in rust_source() which likely have no effect on the compilation process under normal conditions.

• [x] (#114) Invalid example in rust_source(): dependencies accepts list, not string;

  code

  https://github.com/extendr/rextendr/blob/f8f187709497a23f8d1eb65798270cd565fcdd7a/R/source.R#L90-L92

• [x] (#155) Use {glue} string interpolation when formatting cargo args ;

  code

  https://github.com/extendr/rextendr/blob/f8f187709497a23f8d1eb65798270cd565fcdd7a/R/source.R#L266-L274

  Blocked by:

  • https://github.com/tidyverse/glue/issues/100
  • https://github.com/tidyverse/glue/pull/217

• [x] Capture cargo output when compilation fails and produce a meaningful error message;

  code

  https://github.com/extendr/rextendr/blob/f8f187709497a23f8d1eb65798270cd565fcdd7a/R/source.R#L278-L280
  • [x] (#125) Introduce rust_eval_deferred() to separate Rust code compilation from execution
  • [x] Update knitr engines
  • Update to the new {rlang} once it gets released to incorporate this fix https://github.com/r-lib/rlang/pull/1215
  • [x] (#155) Switch to {processx}, capture and parse cargo output.

• [x] (#127) On Windows, path to build directory has back slashes, while every other path formatted with rextendr::pretty_rel_path() has forward slashes. Should be fixed with normalizePath(..., winslash = "/")

  code

  https://github.com/extendr/rextendr/blob/00ce4650ee044e4b7c66ebeaecaa65c6d5ec7024/R/source.R#L113-L117

• [x] (#127) Unused function declaration in rust_eval_deferred()