add GitHub Actions

[eddelbuettel]

Travis CI keeps talking about unlimited use for open source projects yet my runs are currently stalled -- not nice.

So this simple PR turns on GitHub Action with a build matrix just like the one we had before.

No code, documentation or other changes.

[eddelbuettel]

The code coverage change seems spurious. From the look at the .travis.yml we were running it for each of the four gcc/g++ setups (oops, wasteful) whereas I now run it only for gcc-10/g++-10. That should not affect things but we can sort that out another time.

[codecov]

Codecov Report

Merging #61 (254c019) into master (8826fad) will decrease coverage by 8.83%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
- Coverage   95.95%   87.11%   -8.84%     
==========================================
  Files          18        4      -14     
  Lines        1408      194    -1214     
==========================================
- Hits         1351      169    -1182     
+ Misses         57       25      -32     

Impacted Files	Coverage Δ
inst/include/RcppSimdJson/deserialize/scalar.hpp
src/exported-utils.cpp
src/internal-utils.cpp
src/rcppsimdjson_utils_check.cpp
src/simdjson_example.cpp
inst/include/RcppSimdJson/utils.hpp
inst/include/RcppSimdJson/deserialize/simplify.hpp
inst/include/RcppSimdJson/deserialize/matrix.hpp
inst/include/RcppSimdJson/common.hpp
...t/include/RcppSimdJson/deserialize/Type_Doctor.hpp
... and 4 more

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