Adds monad laws and a tool to check them

[sobolevn]

Tada! 🎉

TODOs for later:

• add hypothesis to dev dependencies
• add docs for contrib.hypothesis
• test all possible arguments to check_all_laws
• add laws for applicative and bindable
• add type tests for SupportsEquality
• add tests for .equals()
• add equals to all containers that can be compared
• add type tests for check_all_laws
• test .laws() method resolution for different types
• fix typesafety/test_interfaces/test_iterable/test_inheritance.yml
• add IOBasedN tests

New laws to implement:

• AltableN must have the same laws as MappableN
• .from_failure has a law: https://wiki.haskell.org/Typeclassopedia#MonadFail
• AsyncFutureN has idempotence law
• ReaderBasedN must be pure

Closes #533
Closes #564
Closes #563
Closes #541
Closes #542

[sobolevn]

Ok, it is better to switch to https://hypothesis.readthedocs.io/en/latest/data.html#hypothesis.strategies.builds and create custom strategies for our internal types. We would need https://hypothesis.readthedocs.io/en/latest/data.html#hypothesis.strategies.register_type_strategy for it.

[sobolevn]

I kindly ask @Zac-HD to help me with reviewing hypothesis related parts in this PR.
(Sorry, this PR is huge for reasons 😄 )

Here they are:

• entrypoint (not working for now, see Problems with hypothesis entrypoints HypothesisWorks/hypothesis#2564) https://github.com/dry-python/returns/pull/543/files#diff-0df8bc06f8bf434789b78a0a77398673R1
• our check_all_laws magic function defition: https://github.com/dry-python/returns/pull/543/files#diff-4472972bc0bac0e9127dc675dbd9f29cR1
• Equality laws definition: https://github.com/dry-python/returns/pull/543/files#diff-8aa5189ba9a3be0849ada098c397d9a8R9
• Functor laws definition: https://github.com/dry-python/returns/pull/543/files#diff-e06c48cc820df2c8762959046a397024R21
• Law type and friends: https://github.com/dry-python/returns/pull/543/files#diff-d07e772a790669b34462d39b35073de5R1
• Actual tests: https://github.com/dry-python/returns/pull/543/files#diff-e7433e623c9758b157f27d766eb0df03R8

[Zac-HD]

This is about all I have time for, but nothing stands out as problematic. Good luck!

[sobolevn]

@Zac-HD Thanks a lot for your help! 👍

[codecov]

Codecov Report

Merging #543 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            master      #543    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           77        81     +4     
  Lines         1600      1765   +165     
  Branches       103       122    +19     
==========================================
+ Hits          1600      1765   +165     

Impacted Files	Coverage Δ
returns/_internal/futures/_future.py	100.00% <ø> (ø)
returns/_internal/futures/_future_result.py	100.00% <ø> (ø)
returns/_internal/futures/_reader_future_result.py	100.00% <ø> (ø)
returns/_internal/iterable.py	100.00% <ø> (ø)
returns/_internal/pipeline/flow.py	100.00% <ø> (ø)
returns/_internal/pipeline/managed.py	100.00% <ø> (ø)
returns/interfaces/specific/ioresult.py	100.00% <ø> (ø)
returns/_internal/pipeline/pipe.py	100.00% <100.00%> (ø)
returns/context/requires_context.py	100.00% <100.00%> (ø)
returns/context/requires_context_future_result.py	100.00% <100.00%> (ø)
... and 22 more

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