generalized slice syntax

[mattkretz]

There seems to be a pattern that we need to explore:

We want slices of

• simd,
• contiguous_container (e.g. span, vector), and
• (strided) slices for mdpsan.

More?

Many other languages have syntax to support this. E.g. Python:

a[start:stop]  # items start through stop-1
a[start:]      # items start through the rest of the array
a[:stop]       # items from the beginning through stop-1
a[:]           # a copy of the whole array
a[start:stop:step] # start through not past stop, by step
a[-1]      # last item in the array
a[-2:]     # last two items in the array
a[:-2]     # everything except the last two items
a[::-1]    # all items in the array, reversed
a[1::-1]   # the first two items, reversed
a[:-3:-1]  # the last two items, reversed
a[-3::-1]  # everything except the last two items, reversed

extract<0, 4>(v) could be written as v[0:4] with that syntax.