Add experimental support for automatic fork-join parallelism

[brunocodutra]

This PR adds a feature flag parallel that takes advantage of the experimental support for specialization available on nightly Rust (rust-lang/rust#31844), to automatically parallelize tuples of Sync / Send Reducers using Rayon.

For now it includes a custom implementation of the join! macro that should be replaced by rayon-rs/rayon/pull/406 once it's merged upstream.

[codecov]

Codecov Report

Merging #6 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master     #6   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          14     14           
  Lines         148    182   +34     
=====================================
+ Hits          148    182   +34

Impacted Files	Coverage Δ
src/reducer/mod.rs	100% <ø> (ø)	⬆️
src/store.rs	100% <100%> (ø)	⬆️
src/reactor/slice.rs	100% <100%> (ø)	⬆️
src/reactor/array.rs	100% <100%> (ø)	⬆️
src/reactor/tuple.rs	100% <100%> (ø)	⬆️
src/reducer/rc.rs	100% <100%> (ø)	⬆️
src/reactor/option.rs	100% <100%> (ø)	⬆️
src/macros.rs	100% <100%> (ø)
src/mock.rs	100% <100%> (ø)
src/reducer/arc.rs	100% <100%> (ø)	⬆️
... and 6 more

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