Disable overhead of dead code warnings outside of -Ywarn-dead-code
Remove @inline from mapConserve

The way it is formulated with an inner tail recursive loop,
I don't think it can ever be inlined, and just contributes a lot of
warnings to -Yinliner-warnings.
Fix @inlinability of devWarning

It wasn't inlinable because it called a protected method, `isDeveloper`.
Inlinability for Context#withMode

Finalize it explicitly, and make it effectively final as a backstop
by sealing Context.
Note manually inlined code in atOwner.
Notes with more optimization ideas.
Rewrite List(a, b, c) to `a :: b :: c :: Nil`

Only for small lists, to avoid being too stack hungry.