x509-validation: Export doValidate and no IO

[ch1bo]

Hi,

I am using validate from x509-validation package in an otherwise pure (no IO) scenario. After looking at the code I would be interested in accessing doValidate and pass in the current DateTime or not use that at all (disabling the check).

Would you be open about a PR where doValidate is exported and changed to something like:

doValidate :: Monad m => DateTime
           -> ValidationHooks
           -> ValidationChecks
           -> CertificateStore
           -> ServiceID
           -> CertificateChain
           -> m [FailedReason]

As far as I could tell there is no reason to have this lower-level checking function run in IO.

Thanks for creating these packages!

[infinisil]

I made a PR for this here: #126