improvement: return Rc<RefCell<Runtime>> from builder

[Wodann]

No description provided.

[codecov]

Codecov Report

Merging #153 into master will increase coverage by 0.00%.
The diff coverage is 71.42%.

@@           Coverage Diff           @@
##           master     #153   +/-   ##
=======================================
  Coverage   82.26%   82.26%           
=======================================
  Files         160      160           
  Lines       11022    11028    +6     
=======================================
+ Hits         9067     9072    +5     
- Misses       1955     1956    +1     

Impacted Files	Coverage Δ
crates/mun/src/main.rs	0.00% <0.00%> (ø)
crates/mun_runtime/src/lib.rs	71.87% <71.42%> (-0.17%)	⬇️
crates/mun_runtime/tests/util.rs	90.90% <100.00%> (ø)
crates/mun_runtime_capi/src/lib.rs	96.96% <100.00%> (+0.14%)	⬆️

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[baszalmstra]

Nice ergonomic change