Replace gratuitous memmove() with memcpy()

Replace gratuitous memmove() with memcpy()

The index access methods all had similar code that copied the
passed-in scan keys to local storage.  They all used memmove() for
that, which is not wrong, but it seems confusing not to use memcpy()
when that would work.  Presumably, this was all once copied from
ancient code and never adjusted.

Discussion: https://www.postgresql.org/message-id/flat/f8c739d9-f48d-4187-b214-df3391ba41ab@eisentraut.org