Refactor gas-monad to operate on raw-context

As per comments here it might make sense to move the Gas_monad down the stack so that it operates on Raw_context.t rather than on Alpha_context.context.

We would also expose the current version on top (for instance in Alpha_context.Gas_monad.

That way we can expose lower-level functions, such as Ticket_has_repr.make to return a Gas_monad.t value instead of relying on the Raw_context.t.