Allow instantiation of multiple notebooks inside a single scope

[GordonSmith]

FWIW I am creating notebooks at runtime (rather than build time) and having a single runtime/main is problematic.

[mbostock]

Makes sense; we should generalize this to be more object-oriented. I think the pattern should probably be to instantiate something, an object that owns the runtime and the main module, and then call define on the instantiated object as a method, rather than passing the main module through to define.

[GordonSmith]

That would be even better, was going with a minimal PR.

[GordonSmith]

Updated as suggested (included the ability re-define or undefine a cell, as per our previous discussion)

[mbostock]

Thanks for incorporating my feedback. This looks useful.

For backwards compatibility, I think we’ll want a defaultInstance and retain the existing define, main, runtime symbols (on top of the defaultInstance, which doesn’t need to be exposed directly). We can deprecate these but I’d like to maintain compatibility with the current release.

[mbostock]

I’d also prefer to keep the redefining and stateById logic out of this PR and leave that in userland for now. You should still be able to implement it as described in #26 (comment).

[mbostock]

With the defaultInstance approach, this mutation could be done statically. (But really we should move it to the Runtime itself… Future work!)

[mbostock]

Superseded by #49.