Add support for Rust/Evxcr

[jonasbb]

I tried to follow the listed for "Add support for another language". I do have some questions about the last point though.

Evcxr has special commands, which are not valid Rust in any way. In contrast to Python magic cells they start with : instead of %, so they are not covered by the current logic for line magic.
From the code it looked like changing the variables _MAGIC_RE/_MAGIC_FORCE_ESC_RE might be the way to go here, but since I do not see any special casing there yet, I first wanted to know if that would be correct.

Here is a list of the supported magics in Evcxr.

:vars             List bound variables and their types
:opt [level]      Toggle/set optimization level
:explain          Print explanation of last error
:clear            Clear all state, keeping compilation cache
:dep              Add dependency. e.g. :dep regex = "1.0"
:version          Print Evcxr version
:preserve_vars_on_panic [0|1]  Try to keep vars on panic

Mostly for development / debugging purposes:
:last_compile_dir Print the directory in which we last compiled
:timing           Toggle printing of how long evaluations take
:last_error_json  Print the last compilation error as JSON (for debugging)
:time_passes      Toggle printing of rustc pass times (requires nightly)
:internal_debug   Toggle various internal debugging code

Rust identifiers can also contain : at the start, for example ::std::mem::drop.
The difference is that Evcxr commands have exactly one and in identifiers it is always as ::.

The sample notebook stems from the Evcxr project, which provides a Rust kernel.
https://github.com/google/evcxr/blob/942e166965e47073a592864440acd59cb8f4dfe1/evcxr_jupyter/samples/evcxr_jupyter_tour.ipynb

[codecov]

Codecov Report

Merging #351 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #351      +/-   ##
=========================================
+ Coverage    99.2%   99.2%   +<.01%     
=========================================
  Files          72      73       +1     
  Lines        7031    7052      +21     
=========================================
+ Hits         6975    6996      +21     
  Misses         56      56

Impacted Files	Coverage Δ
jupytext/languages.py	100% <ø> (ø)	⬆️
tests/test_read_simple_rust.py	100% <100%> (ø)
tests/test_mirror.py	100% <100%> (ø)	⬆️
jupytext/magics.py	100% <100%> (ø)	⬆️

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[mwouts]

Awesome! Thank you @jonasbb.

Let me see what I can propose regarding the : magics - maybe we can simply have a rust case in the magics.is_magic function. I'll give it a try, add a few tests, and you will tell me if that does the job.

[mwouts]

Hi @jonasbb , I think we're good: 297cf56. If that sounds right for you I'll merge this into master. And thanks for contributing!

[jonasbb]

@mwouts Thanks for finishing the PR. Looks good.