Refactor wasmi_core::memory submodule

[Robbepop]

This now exposes load, load_at, store and store_at functions additionally to the load_extend and store_wrap ones from the wasmi_core::memory submodule.

This way we no longer require the weird WrapInto and ExtendInto impls for T -> T which made no sense.
Furthermore, this is going to simplify implementation of #1395.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.52%. Comparing base (2b4fa76) to head (02469fb).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1401      +/-   ##
==========================================
+ Coverage   70.48%   70.52%   +0.03%     
==========================================
  Files         157      157              
  Lines       14398    14414      +16     
==========================================
+ Hits        10149    10166      +17     
+ Misses       4249     4248       -1     

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