feat(runtime): add cloning of StructRef

[Wodann]

Apply after #142

[codecov]

Codecov Report

Merging #143 into master will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #143      +/-   ##
==========================================
+ Coverage   81.98%   82.04%   +0.06%     
==========================================
  Files         160      160              
  Lines       10816    10826      +10     
==========================================
+ Hits         8867     8882      +15     
+ Misses       1949     1944       -5     

Impacted Files	Coverage Δ
crates/mun_runtime/src/struct.rs	76.80% <ø> (ø)
crates/mun_runtime/tests/marshalling.rs	100.00% <100.00%> (ø)
crates/mun_memory/src/gc/root_ptr.rs	89.47% <0.00%> (+26.31%)	⬆️

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