Views with an array layout should be indexable with a slice

[whitequark]

Currently this breaks (with a nicer message since c649045 / #1375) but there's no reason this can't work; it should return another ArrayLayout instead:

  File "/home/whitequark/Projects/glasgow/software/test.py", line 35, in elaborate
    m.d.sync += o_reg.eq(o_reg[1:])
                         ~~~~~^^^^
  File "/home/whitequark/Projects/glasgow/software/.venv/lib/python3.12/site-packages/amaranth/lib/data.py", line 796, in __getitem__
    raise TypeError(
TypeError: View cannot be indexed with a slice; did you mean to call `.as_value()` first?

cc @isabelburgos

[whitequark]

The workaround for the pattern above is fairly annoying too:

m.d.sync += o_reg.eq(o_reg.as_value()[len(o_reg[0]):])
# instead of
m.d.sync += o_reg.eq(o_reg[1:])