feat: make the runtime send

[baszalmstra]

Removes the need to put the Runtime into a Rc<RefCell<Runtime>>. This makes the Runtime Send.

Closes #299

[codecov]

Codecov Report

Merging #370 (857bcae) into main (3a481eb) will increase coverage by 0.00%.
The diff coverage is 94.87%.

@@           Coverage Diff           @@
##             main     #370   +/-   ##
=======================================
  Coverage   82.53%   82.54%           
=======================================
  Files         264      264           
  Lines       15287    15247   -40     
=======================================
- Hits        12617    12585   -32     
+ Misses       2670     2662    -8     

Impacted Files	Coverage Δ
crates/mun/src/ops/start.rs	0.00% <0.00%> (ø)
crates/mun_runtime/benches/util/mod.rs	0.00% <0.00%> (ø)
crates/mun_runtime/tests/hot_reloading.rs	100.00% <ø> (ø)
crates/mun_runtime/src/lib.rs	68.65% <66.66%> (-0.16%)	⬇️
crates/mun/tests/integration.rs	100.00% <100.00%> (ø)
crates/mun_memory/src/gc/root_ptr.rs	66.66% <100.00%> (+3.50%)	⬆️
crates/mun_runtime/src/adt.rs	78.70% <100.00%> (+2.70%)	⬆️
crates/mun_runtime/tests/marshalling.rs	98.20% <100.00%> (-0.07%)	⬇️
crates/mun_runtime/tests/memory.rs	100.00% <100.00%> (ø)
crates/mun_runtime/tests/runtime.rs	100.00% <100.00%> (ø)
... and 4 more

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