Use Stack in Executor

[Robbepop]

This replaces the ValueStack and CallStack references with a single Stack reference in the Executor.

[codecov]

Codecov Report

Attention: Patch coverage is 68.62745% with 16 lines in your changes missing coverage. Please review.

Project coverage is 80.47%. Comparing base (3297811) to head (86b9f66).

Files	Patch %	Lines
crates/wasmi/src/engine/executor/instrs/call.rs	71.42%	6 Missing ⚠️
crates/wasmi/src/engine/executor/instrs/return_.rs	61.53%	5 Missing ⚠️
crates/wasmi/src/engine/executor/instrs.rs	62.50%	3 Missing ⚠️
crates/wasmi/src/engine/executor/stack/mod.rs	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1071      +/-   ##
==========================================
- Coverage   80.50%   80.47%   -0.03%     
==========================================
  Files         269      269              
  Lines       24970    24975       +5     
==========================================
- Hits        20101    20099       -2     
- Misses       4869     4876       +7     

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