This commit removes a couple of `execgen.COPYVAL` calls that were
redundant because the first and the second argument are the same. These
calls are redundant because we already performed the same call right
after calling `Get` from the original vector and we will perform a deep
copy when calling `Set` next.

Release note: None

This commit reduces the size of the hash aggregates by removing the
reference to the well-typed column (i.e. a concrete unwrapped
`coldata.Vec`, something like `[]int64`). This is possible because the
hash aggregates only access the concrete column once, in `Flush`, so
there is no point in storing the concrete column as we do for the
ordered aggregates. We still perform the interface dispatch call only
once - previously it was in `SetOutput`, now it is in `Flush`. This
should be a non-trivial improvement since the hash aggregation uses
a separate aggregation function object for each bucket.

This change also allows us to remove the overriding of `SetOutput`
method implimentation provided by the base struct from the hash and
window aggregates.

Release note: None

This commit removes some of the slice element assignments of the form
`col[idx] = ...` in favor of its equivalent `col.Set(idx, ...)`. This
makes it easier to change the type of the element.

Release note: None