Support all atomic types as nullable types

[rvanvelzen]

Fixes #124

[ondrejmirtes]

Perfect! I'm eager to merge this too :)

[ondrejmirtes]

Thank you!

[ondrejmirtes]

FYI I'm gonna try phpdoc-parser in phpstan-src again after this suggestion phpstan/phpstan-src#1406 (comment) - I think it's really necessary to make it optional and non-default because the new behaviour is much stricter and will make a lot of projects fail, so it's a good fit for bleedingEdge (and will allow us to release it in a minor phpdoc-parser version).

[ondrejmirtes]

But I understand it's not trivial. Ideally I'd like to see all current data providers to be tested with both true / false and only those that differ to be tested in a different test case method probably.

[rvanvelzen]

I'll hopefully get to it tomorrow :)