Make IsolationLevel pub

[Dhghomon]

Resolves #291 in which a user noticed that IsolationLevel is required for TransactionOptions::isolation but didn't make the pub use statement for the rest of the types in the options module.

[elprans]

I think we should just remove/unpublish the isolation method. It's not useful, because serializable is the only isolation option.

[Dhghomon]

I think we should just remove/unpublish the isolation method. It's not useful, because serializable is the only isolation option.

That would work. Should we remove the IsolationLevel enum as well or are there plans to add to it in the future?

[aljazerzen]

I've removed isolation method and IsolationLevel itself. Here is justification from commit message:

At the moment, isolation level of a transaction can only be
SERIALIZABLE. It can be specified when opening a connection, but
if it is not, it will default to SERIALIZABLE.

There is also no plans to support other level soon.

IsolationLevel thus serves no purpse and also cannot be set outside
of this crate, since it is not public.

So there is no harm in removing it. When/if we implement other
levels, we can revert this commit.

[CodesInChaos]

If you later re-add it (e.g. because you implement snapshot readonly support), you should mark the enum as #[non_exhaustive], so adding more options isn't a breaking change.