Review `Sandbox.run()` API with regards to batching

Currently the batching of commands in element plugins is explicit, however unbatched commands are suboptimal in a remote execution context.

In some cases, the result of executed commands may inform the plugin as to what the following commands might be, so probably we need support for unbatched commands too.

Related to this is the possibility of dropping the split between the prepare() method and the assemble() method.

https://mail.gnome.org/archives/buildstream-list/2020-April/msg00013.html

Also consider using the scripts we generate for command batching as replacement for the separate Element.generate_script() (bst source checkout --include-build-scripts).