Add new instance cache: CachedInstance

[Robbepop]

This new instance cache works very different from the old one and is supposedly way more efficient.
It relies on the executor updating it whenever the currently used Instance has been changed. While the executor is in control over that the CachedInstance provides an efficient way to access Instance related data.

[codecov]

Codecov Report

Attention: Patch coverage is 93.20388% with 7 lines in your changes missing coverage. Please review.

Project coverage is 80.50%. Comparing base (82d96f3) to head (69c0f01).
Report is 1 commits behind head on master.

Files	Patch %	Lines
crates/wasmi/src/engine/executor/cache.rs	94.54%	3 Missing ⚠️
crates/wasmi/src/engine/executor/instrs.rs	72.72%	3 Missing ⚠️
crates/wasmi/src/engine/executor/stack/calls.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1076      +/-   ##
==========================================
+ Coverage   80.47%   80.50%   +0.03%     
==========================================
  Files         269      269              
  Lines       24974    24992      +18     
==========================================
+ Hits        20097    20120      +23     
+ Misses       4877     4872       -5     

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