This PR contains NO semantic changes at all, this simply refactors
existing INSERT code to be easier to reason about.
Very sorry I know I've been working on `INSERT OR IGNORE|REPLACE|etc..`
for days now but the insert translation was literally unbearable and I
got it working but I barely could wrap my head around that whole
`translate_insert` function, so I spent a bunch of time refactoring the
whole INSERT handling out into different "Plans"... which turned into a
whole different clusterf***... So I just went back and made the existing
insert emission more modular and created some context that can make it
easier to reason about.
This should be able to just be merged quickly
Reviewed-by: Jussi Saurio <jussi.saurio@gmail.com>
Closes#3731