Specialize __foldl__ for Tuple

[tkf]

There is a very strange test failure I cannot reproduce locally

test_examples_primes.jl: Test Failed at /home/travis/build/tkf/Transducers.jl/examples/primes.jl:27
  Expression: primes == [2, 3, 5, 7]
   Evaluated: [2, 3, 4, 5, 7] == [2, 3, 5, 7]

see, e.g., https://travis-ci.com/tkf/Transducers.jl/jobs/215961196#L530-L532

[codecov-io]

Codecov Report

Merging #17 into master will not change coverage.
The diff coverage is 50%.

@@           Coverage Diff           @@
##           master      #17   +/-   ##
=======================================
  Coverage   83.95%   83.95%           
=======================================
  Files          14       14           
  Lines        1103     1103           
=======================================
  Hits          926      926           
  Misses        177      177

Impacted Files	Coverage Δ
src/core.jl	65.05% <0%> (-0.19%)	⬇️
src/processes.jl	94.04% <100%> (+0.03%)	⬆️

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