Resolve block chains against the render context - #4925

While working on the Symfony PR for #4917, I realize that the performance was worse with the Twig's way. #4924 fixes part of the performance "regression". This one closes the gap.

BlockChain currently freezes each template's lineage by cloning it, so the block map and parent() both resolve {% extends %} against the constructor context.

This PR changes that to resolve the chain against the render context instead. hasBlock() and getBlockNames() take a context, like TemplateWrapper already does, and the third constructor argument becomes a default the render context can override. A lineage whose templates all have a fixed parent (none, or constant) is resolved once and cached, so the common case costs nothing; a dynamic {% extends %} re-resolves per call (not use by Symfony anyway).

This fixes also an inconsistency: a chained template with a dynamic parent now behaves exactly as it does when rendered directly. It also removes all changes in the "core" logic of Template.

                       construct    render   1 chain + 40 renders
Symfony's engine today   13.44us    0.78us         50.3us
#4917                    ~70us      0.96us        ~117us
this PR                   0.61us    0.84us         53.0us