WIP: upgrade async-std to 1.6 beta

[GopherJ]

No description provided.

[GopherJ]

wait to solve: async-rs/async-std#766

[dignifiedquire]

I just checked and this passes all tests when including async-rs/async-std#768

[GopherJ]

@dignifiedquire Thanks, I'll wait for it to be merged then!

[codecov]

Codecov Report

Merging #136 into master will decrease coverage by 4.04%.
The diff coverage is 61.95%.

@@            Coverage Diff             @@
##           master     #136      +/-   ##
==========================================
- Coverage   85.75%   81.71%   -4.05%     
==========================================
  Files          19       20       +1     
  Lines        3025     3111      +86     
==========================================
- Hits         2594     2542      -52     
- Misses        431      569     +138     

Impacted Files	Coverage Δ
src/cache/default_cache.rs	0.00% <0.00%> (-93.34%)	⬇️
src/cached_enforcer.rs	0.00% <0.00%> (-5.69%)	⬇️
src/emitter.rs	0.00% <0.00%> (-66.67%)	⬇️
src/error.rs	100.00% <ø> (ø)
src/logger/default_logger.rs	0.00% <0.00%> (ø)
src/model/function_map.rs	94.64% <0.00%> (-2.20%)	⬇️
src/util.rs	97.77% <0.00%> (ø)
src/internal_api.rs	31.00% <16.32%> (-16.06%)	⬇️
src/convert.rs	40.00% <33.33%> (-10.00%)	⬇️
src/enforcer.rs	85.38% <74.10%> (-5.24%)	⬇️
... and 12 more

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[GopherJ]

closed in favor of #137 , the only difference between #137 and #136 should be async-std, we'll need to wait async-std to have a stable version of 1.6.0 and we also need to find a correct way for writting benchmark