Avoid an iteration of the argument list.
Merge atOwner in SuperAccessors and TypingTransformers

SuperAccessors overrode this method to maintain a map from
class symbol to a Typer, with a note cautioning against doing this
in all TypingTransformers at penalty of 5% performance degradataion.

But we can afford to do this universally if we push them onto a List,
rather than into a Map. By doing this, we can make make `atOwner`
@inline final in the superclass.

Not sure if this offers much of a win, but it flattens out the stack
traces a bit which at least makes profiles a bit easier to navigate.
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.