Remove the downcast-rs dependency from wasmi_core

[Robbepop]

Closes #1513.
Blocked by #1518.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 10 lines in your changes missing coverage. Please review.

Project coverage is 70.68%. Comparing base (f4c321e) to head (a80398a).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/core/src/host_error.rs	0.00%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1517      +/-   ##
==========================================
- Coverage   70.74%   70.68%   -0.07%     
==========================================
  Files         176      177       +1     
  Lines       16575    16585      +10     
==========================================
- Hits        11726    11723       -3     
- Misses       4849     4862      +13     

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