don't reexport discojs-core and tfjs #638
Description
currently, discojs-node & discojs-web are reexporting discojs-core, which looks weird to me: users are mixing imports from both packages & commonization of deps is non-trivial.
having every project importing directly from discojs-core and only platform specifics from discojs-{node,web} makes more sense IMO.
tfjs is also reexported everywhere and that might produce subtle bugs when dealing with mixed versions.
removing this tight coupling would help both dev and users of disco.