Add poc mmap data sharing

[gaborcsardi]

Unix only. Uses https://github.com/r-lib/processx/pull/201

This is really a crude poc now. Notes:

• callr::r supports the transter = "mmap" option now, to transfer all arguments via shared memory.
• it fails if one argument is not an integer, real, raw or logical vector.
• it changes how the subprocess runs its R code: it puts a run() function into a callr environment on the search path. This allows more flexible subprocess code.
• the result is still copied back via a file.
• it needs a better way of passing the fd of the memory mapped file. Right now it just assumes 3L, but that might change depending on whether we have a poll connection or not, or possibly other connections.

[codecov-io]

Codecov Report

Merging #109 into master will increase coverage by 10.09%. The diff coverage is 100%.

@@             Coverage Diff             @@
##           master     #109       +/-   ##
===========================================
+ Coverage   74.39%   84.48%   +10.09%     
===========================================
  Files          20       20               
  Lines        1070      986       -84     
===========================================
+ Hits          796      833       +37     
+ Misses        274      153      -121

Impacted Files	Coverage Δ
R/package.R	0% <ø> (ø)	:arrow_up:
R/eval-bg.R	100% <ø> (ø)	:arrow_up:
R/eval.R	100% <ø> (ø)	:arrow_up:
R/check.R	100% <100%> (ø)	:arrow_up:
R/run.R	100% <100%> (ø)	:arrow_up:
R/r-session.R	74.14% <100%> (+16.51%)	:arrow_up:
R/setup.R	93.28% <100%> (-2.69%)	:arrow_down:
R/script.R	96.2% <100%> (+6.66%)	:arrow_up:
R/options.R	100% <100%> (ø)	:arrow_up:
... and 7 more

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