Add "thenCheck()" to CompiledRepository.

[maxtroy]

"thenCheck()" can be used after another 'then' directive, which normally generates the new value of the repository. Using a "thenCheck()" directive, the would-have-been new value becomes a mere "candidate new value", and can be vetoed by the check -- the clause that follows "thenCheck()" can skip the update, end with another value, or even have the flow continue onwards.

The test cases use two demo-style repositories that perform a typical task of requesting a keyed item from the in-memory cache, then disk cache, then network, showcasing the real-life use of this new directive.

[codecov-io]

Current coverage is 97.38% (diff: 100%)

Merging #113 into master will increase coverage by 0.05%

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  Misses           13         13          
  Partials         30         30          

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