Add operator[] for indexed word access

[chfast]

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[codecov-io]

Codecov Report

Merging #212 (fcfe5d7) into master (dd1de8b) will decrease coverage by 0.06%.
The diff coverage is 99.03%.

@@             Coverage Diff             @@
##            master     #212      +/-   ##
===========================================
- Coverage   100.00%   99.93%   -0.07%     
===========================================
  Files            9        9              
  Lines         1639     1615      -24     
===========================================
- Hits          1639     1614      -25     
- Misses           0        1       +1     

Flag	Coverage Δ
32bit	99.93% <99.03%> (-0.07%)	⬇️
gcc	99.93% <99.03%> (-0.07%)	⬇️

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Impacted Files	Coverage Δ
test/unittests/test_intx_api.cpp	100.00% <ø> (ø)
include/intx/intx.hpp	99.74% <98.57%> (-0.26%)	⬇️
include/intx/int128.hpp	100.00% <100.00%> (ø)
test/experimental/add.cpp	100.00% <100.00%> (ø)
test/unittests/test_intx.cpp	100.00% <100.00%> (ø)