✨ Support type guards while filtering

[dubzzz]

Why is this PR for?

Support type guards while filtering
Fixes #545

In a nutshell

✔️ New feature
❌ Fix an issue
❌ Documentation improvement
❌ Other: please explain

(✔️: yes, ❌: no)

Potential impacts

None expecting. We need to confirm it will not break any existing typings.

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[dubzzz]

I still need to see what's the best solution to handle https://github.com/dubzzz/fast-check/pull/555/files#diff-e874ec8f9333a86a7d38aaab01e2fa59R79-R81

As of today, it may produce invalid typings when calling filter on a shrinkable. Nonetheless, it should not be a huge issue as it will only occured when passed a refinement with an explicit value is U.

[coveralls]

Coverage increased (+0.005%) to 96.316% when pulling f72cb20 on feat/refinement-filter into 1ceb2b9 on master.

[dubzzz]

Green run in Travis - https://travis-ci.org/dubzzz/fast-check/builds/655993341