Move _Result and friends to new module

[goodboy]

Originally proposed (and accepted by moi) in #227.

Relates to:

• fits with discussion in hookwrappers can't modify the in-flight exception without skipping later hookwrapper teardowns #244 the solution in Handle exceptions in hookwrapper post stage (#244) #257 and further ideas and discussion in _Result is unnecessary? #260
• allows for solving a import cycle in Cythonize call loop #104

[goodboy]

Huh, not entirely sure what's up with codecov drop but..