Restore MartenOps.Store and add MartenOps.StoreMany

[jay-zahiri]

Fixes #1162

[davidst]

I suggest to also add some unit tests to verify the correct behavior.

public recod Foo;

public void StoreManyDocs_test()
{
    var docs = new[] { new Foo(), new Foo() };

    var storeOp = MartenOps.StoreManyDocs<Foo>(docs);

    storeOp.Documents.ShouldBe(docs);
}

[jay-zahiri]

I think it's a bit overkill to test the language itself, but you're welcome to add that. Here is a quick update on the demo, if you're unsure.

[davidst]

I agree that it is overkill to test the language itself, but a test for this has nothing to do with testing the language itself. A bug sneaked into the implementation, therefore a test to verify the correct implementation of the StoreManyDocs would be appropriate.