Move wasmi_core memory utilities into its own memory submodule

[Robbepop]

This is going to be useful for #1395.

[codecov]

Codecov Report

Attention: Patch coverage is 93.18182% with 3 lines in your changes missing coverage. Please review.

Project coverage is 70.50%. Comparing base (a0d8b46) to head (95be6ba).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/core/src/memory.rs	93.18%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1400      +/-   ##
==========================================
- Coverage   70.51%   70.50%   -0.01%     
==========================================
  Files         156      157       +1     
  Lines       14397    14398       +1     
==========================================
  Hits        10152    10152              
- Misses       4245     4246       +1     

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