WIP(feat): Store Isolate inside API struct which can be reused

[Auspicus]

Having the library user keep track of the Isolate instance improves performance by 5x from my tests in the benches. This could be further improved by multi-threading since the user can create a new Isolate (SSREnvironment) for each thread. We need to figure out an example to provide. There's a few more performance tweaks to be had:

• [ ] Reusing CachedData for script compilation
• [ ] Reusing Context

[codecov-commenter]

Codecov Report

Merging #8 (3078924) into main (09a13e0) will increase coverage by 9.19%. The diff coverage is 97.91%.

@@            Coverage Diff             @@
##             main       #8      +/-   ##
==========================================
+ Coverage   89.28%   98.48%   +9.19%     
==========================================
  Files           2        2              
  Lines          84       66      -18     
==========================================
- Hits           75       65      -10     
+ Misses          9        1       -8     

Impacted Files	Coverage Δ
src/ssr.rs	97.22% <97.14%> (+13.88%)	:arrow_up:
tests/lib_check.rs	100.00% <100.00%> (ø)

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[Auspicus]

As we discussed, using this in a multi-threaded environment is actually quite difficult because of the way that async runtimes (ie. tokio) handle requests. There's no guarantee that the thread that starts handling a request will follow it all the way through to response if there's a yield as another thread from the thread pool might pick it up after the yield. Each v8 Isolate requires Isolate::Enter and Isolate::Exit to be called when accessed by a new thread. We need to figure out how to handle that :o