sim: Simplify `slot` parameters

Many places in the image code pass a pair of

    slots: &[SlotInfo], slot: usize,

around as arguments.  Simplify this by just passing a `&SlotInfo`.  For
the few instances where code needs to know the index of a slot, add an
index field to the `SlotInfo` struct for these to use.  This eliminates
an argument from 4 functions/methods, and the numerous places they are
called.

Signed-off-by: David Brown <david.brown@linaro.org>