Slight bootstrap re-organization

[Tony Garnock-Jones]

Brian Rice wrote:
> I've updated this again, so that post-bootstrap.slate calls a method
> "start" on lobby which prelude.slate defines as "REPL start". This is
> obviously adjustable, so that, e.g. the SDL UI or a socket server could
> be started instead.

This sounds good.

I'm off on holiday for a month (all of February) - I'm hoping to have a
bit more energy (and less RSI) when I get back!

Tony