Loosen the reentrance rules a bit for parent/child scenarios

[lukewagner]

This PR loosens the reentrance rules to allow canon lift to be called from inside a component instance such as when a parent component is called by a child component. For example, in this example:

(component
  (core module $Core1 ...)
  (core instance $core1 (instantiate $Core1))
  (func $func1 (canon lift (core func $core1 "func")))
  (component $Child ...)
  (instance $child (instantiate $Child (with "func1" (func $func1))))
  (core func $child_func (canon lower (func $child "func")))
  (core module $Core2 ...)
  (core instance $core2 (instantiate $Core2 (with "child" (instance (export "func" (func $child_func))))))
  (func $func2 (canon lift (core func $core2 "func")))
  (export "func2" (func $func2))
)

as previously written, $func2 would clear may_enter so that $func1 would trap when called by $child. With this PR, the idea is that $func1 wouldn't check may_enter because called_as_export would be False. A more realistic example of when you'd want to do this is that the parent component uses $Core1 and $Core2 to virtualize a filesystem for $Child, including passing preopens into $Child's exports.

One consequence of this relaxation is that same-component-instance lift+lower would be possible and such cases would need to be compiled more pessimistically (although in a way that only impacted this (rare) case). The PR explains more.

[alexcrichton]

Seems reasonable to me, and to confirm, it's possible to optimize a same-component lift/lower if the linear memories they reference are provably disjoint, right?

[lukewagner]

Yep!