Avoid unnecessary copies of AVIF primary_item and alpha_item

When they're already contiguously present in an mdat buffer (even the same one),
AvifContext::primary_item() and ::alpha_item() can return slices rather than
having to copy the (potentially large) buffers.

This results in a measured performance increase of ~50% for parsing a 4.7 MB
AVIF (the largest in our test set):

$ git checkout master
$ cargo bench -- --save-baseline base
avif_largest            time:   [2.0990 ms 2.1352 ms 2.1727 ms]

$ git checkout avif-nocopy
$ cargo bench -- --save-baseline avif-nocopy
$ cargo bench -- --load-baseline avif-nocopy --baseline base
avif_largest            time:   [1.0077 ms 1.0257 ms 1.0449 ms]
                        change: [-52.649% -51.371% -50.022%] (p = 0.00 < 0.05)
                        Performance has improved.

Also, clean up some related code add add tests.

Author: baumanj