Make Agents / Agent Clusters concepts that work for WebAssembly

[annevk]

See the three comments starting at #882 (comment). We need an abstract super concept that holds the SharedArrayBuffer data and then subclasses for ECMAScript and WebAssembly. It would probably be best for that super concept to be defined in ECMAScript since it also defines most of the other details.

See WebAssembly/design#1019 for the corresponding (informal) work in WebAssembly.

cc @jfbastien @syg @binji

[annevk]

See also #1357.