Add an example: estimating pi using Monte-Carlo method

[tkf]

Commit Message

Add an example: estimating pi using Monte-Carlo method (#26)

Add an example for combining FoldsCUDA and counter-based RNG in
Random123. This requires minor fixes in FoldsCUDA and Transducers
that was expecting the length(_) to be an Int which is not the
case for StepRange{UInt64, UInt64} returned by counters(n) used in
the example.

[codecov]

Codecov Report

Merging #26 into master will decrease coverage by 2.37%.
The diff coverage is 46.15%.

@@            Coverage Diff             @@
##           master      #26      +/-   ##
==========================================
- Coverage   66.23%   63.85%   -2.38%     
==========================================
  Files           7        9       +2     
  Lines         154      166      +12     
==========================================
+ Hits          102      106       +4     
- Misses         52       60       +8     

Impacted Files	Coverage Δ
examples/monte_carlo_pi.jl	36.36% <36.36%> (ø)
src/kernels.jl	49.52% <100.00%> (-0.96%)	⬇️
test/nogpu/test_monte_carlo_pi.jl	100.00% <100.00%> (ø)

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[tkf]

CPU/threading requires JuliaFolds/Transducers.jl#433