FIX-896: product type in algo support

[DenisYaroshevskiy]

• No more traits controlling the type in preprocess range. The way to go is to eve::algo::convert(rng, eve::as<T>{})
• Now we can do the basic conversions for zip iterators, at least from flat to a complex product type. Enough for soa_vector
• Supported passing constants in reduce as zero. But not as init, that's a bit trickier. Will fix in: [BUG] type selection for inclusive_scan/reduce #938

Closes: #880, and #896