Change shift amount param from unsigned to uint64_t

[chfast]

This simplifies reduction from any intx::uint to the shift amount param
because we can just use the lowest word for this.

[codecov-io]

Codecov Report

Merging #202 (fbbf837) into master (6c39eb7) will decrease coverage by 0.23%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #202      +/-   ##
==========================================
- Coverage   99.88%   99.65%   -0.24%     
==========================================
  Files           9        9              
  Lines        2567     2579      +12     
==========================================
+ Hits         2564     2570       +6     
- Misses          3        9       +6     

Flag	Coverage Δ
32bit	99.87% <100.00%> (-0.13%)	⬇️
clang	99.09% <100.00%> (-0.26%)	⬇️
gcc	99.87% <100.00%> (-0.13%)	⬇️

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Impacted Files	Coverage Δ
include/intx/int128.hpp	99.46% <100.00%> (ø)
include/intx/intx.hpp	99.19% <100.00%> (-0.81%)	⬇️
test/unittests/test_intx.cpp	100.00% <100.00%> (ø)