Swap BC layer for yield-ready and reclaim perf loss

[nicolas-grekas]

Follows #3999

Fix #4146
Fix #4103

When use_yield is set to false (the default), this PR reverts the implementation of the render() method to use a wrapping output buffer instead of hooking between each steps of generators. In this mode, the behavior of the yield method is not "pure": it triggers a mix of yield and echo. But this is fine for render and display methods.

When use_yield is set to true, we skip that wrapping output buffer. This makes twig compatible with fibers (and this also makes compilation fail if a non-YieldReady extension is found.)

That makes the name of the option not ideal, but BC rulez FTW.

[fabpot]

@nicolas-grekas Can you add a note in CHANGELOG?

[fabpot]

I've just tested on https://github.com/j0r1s/twig-performance-loss-reproducer, and it indeed fixes the issue with the default use_yield => false configuration:

Before: https://blackfire.io/profiles/a94a955c-b84d-4dfb-94f0-89300e0aa4a3/graph
After: https://blackfire.io/profiles/bde4be25-00da-432e-87a0-087ed3ab5e37/graph
Comparison: https://app.blackfire.io/profiles/compare/bde4be25-00da-432e-87a0-087ed3ab5e37...a94a955c-b84d-4dfb-94f0-89300e0aa4a3/graph?settings%5Bdimension%5D=wt&settings%5Bdisplay%5D=landscape&settings%5BtabPane%5D=nodes&selected=&callname=main()&constraintDoc=

I've also tested on a simpler test script and it works great as well.

[fabpot]

Thank you @nicolas-grekas.

[fabpot]

Oops, forgot about the CHANGELOG. Fixed here: 3a307cd

[smnandre]

Wow thank you @nicolas-grekas for this! 👏 🙇