Bump to rust edition 2021 + best practices.

[Grinkers]

The async feature has been removed.
The async example is now using stable async/futures.

Relevant discussions:
#116
#117

I'm not sure how many people use this crate, but I think this might be one of the last big breaking changes for standard features. A 0.18.0 release might be a good idea. An added note about an org change (#109) from 0.19.0 might be a good idea.

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (fe6b043) 71.38% compared to head (cffc902) 71.53%.

Files	Patch %	Lines
src/edn/utils/index.rs	0.00%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #118      +/-   ##
==========================================
+ Coverage   71.38%   71.53%   +0.15%     
==========================================
  Files           8        8              
  Lines         940      938       -2     
==========================================
  Hits          671      671              
+ Misses        269      267       -2     

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[naomijub]

I will bump to 0.18
We should decide the org name

[Grinkers]

@naomijub hold that release for #119
hopefully the last breaking change I suggest