Proposal: make FsCheck.Experimental.Operation.Check return 'Testable instead of Property.

[teo-tsirpanis]

It would allow better integration with those who prefer Asserting with their test libraries, instead of using FsCheck properties.

[kurtschelfthout]

See #541

Note that I threaded the 'Testable through all types. So you can't use bool in one Operation and unit in another now. I'm not 100% convinced that is better.