`reduce_core` was previously forced to clone the output data before
presenting it to the logic function for inspection.

Simply annotating the type with a reference was not possible due to the
buffer being held in the same struct as some of the data it contained,
making it a self-referencial struct which the borrow checker doesn't
allow.

However, the only reason this buffer was held in the same struct was to
reuse its allocation. This patch therefore creates a brand new vector
every time we need to process data that contains references instead of
cloned data. The expectation is that in the general case it's better to
allocate a vector of references than clone a bunch of values.

Signed-off-by: Petros Angelatos <[email protected]>

This patch brings back the benefits of re-using the output buffer
allocation at the cost of some unsafe code reasoning.

Since the buffer is always cleared after each use we can simply store
its type erased pointer and temporarily bring it back to its Vec form
whenever there is some processing to do.

Signed-off-by: Petros Angelatos <[email protected]>