Add Rust Engine

[TianyiShi2001]

Adding support for running Rust code in ```{rust} and ```{cargo} chunks.

```{rust} chunks call rustc, while ```{cargo} chunks call cargo run. The main difference is that, ```{cargo} chunks allow using external crates, but this also comes at a cost of additional configuration (see below).

```{rust} implementation details:

This is achieved by copying code into a temporary file foo.rs and then run rustc foo.rs && ./foo.

You may need to specify the path to rustc in ~/.Renviron because Sys.getenv('PATH') in R is different from echo $PATH in shell.

Demo:

You can even omit fn main() in simple cases:

(this is achieved by first grepping 'fn main', and if it's not found, wrapping your code with fn main(){ and }

```{cargo} implementation details:

1. the user sets options(knitr.cargo.path= "PATH_TO_CARGO"), where PATH_TO_CARGO is the directory that contains Cargo.toml Please suggest a better name for knitr.cargo.path!
2. eng_rust_cargo() extracts the cargo directory using getOption("knitr.cargo.path")
3. the content of each ```{cargo}block is copied to CARGO_DIR/src/main.rs (fn main() {} added if absent)
4. setwd() to the cargo directory, and cargo run -q is executed by system2(), then wd is reset

Demo:

[yihui]

Looks good to me! It'll be great if you could use <- or = consistently (personally I prefer the latter). Thank you!

[CLAassistant]

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[cderv]

FWIW I just saw there is a rust knitr engine in the R package extender

https://github.com/extendr/rextendr

Not yet on CRAN but part of a rust for R ecosystem. A good home for such an engine.

[yutannihilation]

Feedback is welcome! R Markdown-related usages can be found here:

https://extendr.github.io/rextendr/articles/rmarkdown.html

[yihui]

The extendr package looks much more promising than calling the command directly via system2() in R, so I'll close this PR and recommend users to try extendr instead. Thanks!