Rmlx 0.3.0

• We now allow float64 arrays. (Note that mlx doesn't yet do operations
  on the gpu for these, however.)
• Breaking change: arrays no longer have an associated device. Instead,
  use mlx_device(), with_device() or local_device() to choose a
  device to work on. This also makes irrelevant a bug in which
  many/most operations were silently ignoring their operands' device.
• Operations which are currently cpu-only used to switch silently
  to cpu. They now take a device argument which must be explicitly
  set to "cpu". This is a teaching tool to help the user understand what
  operations can take place on which device.
• rbind() and cbind() now accept 1D vectors.
• Bugfix: mlx_expand_dims() works with 1D vectors.