Change checkpoint to be `checkpoint(name, tags...; data...)`

As of #15 (cc @mzgubic)
we are allowed duplicate tags, and because of this `with_tag` is used as `with_tag(:tag1=1, :tag2=2) do`
Since we need to actually have a function that can take those duplicate tags they can't be in the keyword positon.

Conversely, the data keys must be unique, so having them in the varargs position is suboptimal.

Also as of #15 it will be very rare to pass tags directly to a checkpoint.
So calls will be `checkpoint("FooBar"; foo=1, bar=[1,2,3])` which will create JLSO files containing both `foo` and `bar`.
In a location determined by the context tags