Nested validation and AsyncXHP bugfix

This release fixes an issue where ❌composable-element would not fully
process nested structures, except for rendering them. In particular,
while children were always validated for root elements, they would not
always be validated for deeper nested children.

This release may uncover bugs in code that depends on children not being
validated.

This has been fixed by combining the render and validation steps into
one recursive process - this may change the execution order of AsyncXHP
elements, however this is already undefined by HHVM.