src: cache and resue isolate and contex pointers
Many places these values are obtained through env
pointer that incurs function calls. Source in once and re-use as much as possible. There are more of this pattern in this file, but those are either
one-time use or used in conditional blocks which means sourcing-in those data early may not be beneficial.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines