SSR: eliminate closure in render.js implementation

[HerringtonDarkholme]

I'm trying to remove closures in src/server/render.js.

Removing closures can improve runtime performance, enable easier reasoning about memory leakage, and prevent erroneous captured variable reused.

The basic idea is to pack all captured variable into a RenderContext structure. In closure based implementation, all state changes are implicitly stored by wrapping next function. In context based implementation state transitions are explicitly recorded in renderStates property.

I have ran a preliminary benchmark here by running npn run bench:ssr.
It seems closure elimination can achieve 20% performance boost.
#4002 has also been fixed in this, because contexts are separated between two render function.

[yyx990803]

Wow, very impressive! This is a pretty substantial PR, so I'll take a bit more time to evaluate it, but at a glance seems very well done.

[HerringtonDarkholme]

Thanks for quick response! Leave comments if you have suggestions, I'm happy to fix.

[yyx990803]

Manually resolved and merged as 2ac581b - thanks! Very nice work.