Export deserialization function to both C++ and R (closes #80)

[eddelbuettel]

This PR turns on exporting at the C++ and R level for the source file containing the deserialize_json internal workhorse function. No change at the R level, or how the package works internally, but simply offering other packages to utilize this.

Such cross-package use at the C(++) is currently not all that widely used but very powerful. We do not expect any side-effects based on similar use in another package.

[codecov]

Codecov Report

Base: 99.12% // Head: 99.12% // No change to project coverage 👍

Coverage data is based on head (e5544e9) compared to base (78a2233).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #81   +/-   ##
=======================================
  Coverage   99.12%   99.12%           
=======================================
  Files          18       18           
  Lines        1375     1375           
=======================================
  Hits         1363     1363           
  Misses         12       12           

Impacted Files	Coverage Δ
src/deserialize.cpp	100.00% <ø> (ø)

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