Adds do-notation

[sobolevn]

I've decided that it is better to ship something than nothing 🙂
There are several things to do:

• RequiresContext is special (second arg is not an error type)
• RequiresContextResult support
• RequiresContextFutureResult is also special, because we need async for as Future (but a bit different, because RequiresContextFutureResult is created as RequiresContextFutureResult(async def some(deps) -> Result[val, err]))

But, this is usable as-is for now! 🎉
Refs #392

[codecov]

Codecov Report

Merging #1298 (81cb3d7) into master (d271806) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 81cb3d7 differs from pull request most recent head 87c1a46. Consider uploading reports for the commit 87c1a46 to get more accurate results

@@            Coverage Diff            @@
##            master     #1298   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           79        79           
  Lines         2434      2487   +53     
  Branches       211       212    +1     
=========================================
+ Hits          2434      2487   +53     

Impacted Files	Coverage Δ
returns/context/requires_context.py	100.00% <ø> (ø)
returns/future.py	100.00% <100.00%> (ø)
returns/io.py	100.00% <100.00%> (ø)
returns/maybe.py	100.00% <100.00%> (ø)
returns/result.py	100.00% <100.00%> (ø)

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