Serialization support

[gaborcsardi]

This is not ready yet. Raw vectors are done, but the interesting part will be serializing to mmap-ed memory and back.

[codecov-io]

Codecov Report

Merging #216 into master will increase coverage by 1.45%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #216      +/-   ##
==========================================
+ Coverage   70.22%   71.67%   +1.45%     
==========================================
  Files          31       38       +7     
  Lines        2556     3838    +1282     
==========================================
+ Hits         1795     2751     +956     
- Misses        761     1087     +326

Impacted Files	Coverage Δ
src/init.c	100% <ø> (ø)	⬆️
R/serialize.R	0% <0%> (ø)
src/serialization.c	0% <0%> (ø)
src/client.c	36.45% <0%> (-9.84%)	⬇️
src/create-time.c	68.57% <0%> (-3.66%)	⬇️
src/win/utils.c	0% <0%> (ø)
src/win/stdio.c	70.91% <0%> (ø)
... and 8 more

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