Remove Item type

Remove Item type

This type is just char * underneath, it provides no real value, no
type safety, and just makes the code one level more mysterious.  It is
more idiomatic to refer to blobs of memory by a combination of void *
and size_t, so change it to that.

Also, since this type hides the pointerness, we can't apply qualifiers
to what is pointed to, which requires some unconstify nonsense.  This
change allows fixing that.

Extension code that uses the Item type can change its code to use
void * to be backward compatible.

Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Reviewed-by: Peter Geoghegan <pg@bowt.ie>
Discussion: https://www.postgresql.org/message-id/flat/c75cccf5-5709-407b-a36a-2ae6570be766@eisentraut.org