Adds task chains

[Koed00]

• adds the Chain class
• adds the async_chain command
• adds Iter class

[codecov-io]

Current coverage is 96.36%

Merging #96 into master will increase coverage by +0.16% as of 62abeda

@@            master     #96   diff @@
======================================
  Files           39      39       
  Stmts         2449    2560   +111
  Branches         0       0       
  Methods          0       0       
======================================
+ Hit           2356    2467   +111
  Partial          0       0       
  Missed          93      93       

Review entire Coverage Diff as of 62abeda

Powered by Codecov. Updated on successful CI builds.

[devdlabs]

Chains of tasks seems to be stuck up when I try to get result. Any special configuration needed?