test(memory): add type conversion test

[Wodann]

Adds a test to make sure type conversion works when memory mapping.

[codecov]

Codecov Report

Merging #164 into master will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #164      +/-   ##
==========================================
+ Coverage   82.85%   82.89%   +0.03%     
==========================================
  Files         163      163              
  Lines       11221    11223       +2     
==========================================
+ Hits         9297     9303       +6     
+ Misses       1924     1920       -4     

Impacted Files	Coverage Δ
crates/mun_runtime/tests/memory.rs	100.00% <100.00%> (ø)
crates/mun_memory/src/mapping.rs	94.91% <0.00%> (+1.69%)	⬆️
crates/mun_memory/src/diff.rs	96.00% <0.00%> (+2.00%)	⬆️

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