Fix possible race condition in PrecompiledScriptReducer

[aslesarenko]

Closes #778

In this PR:

• mutable access to IR is protected by ReentrantLock
• improved assertion reporting

[codecov]

Codecov Report

Merging #793 (60c5e7b) into master (44253ac) will decrease coverage by 0.09%.
The diff coverage is 40.62%.

@@            Coverage Diff             @@
##           master     #793      +/-   ##
==========================================
- Coverage   66.53%   66.43%   -0.10%     
==========================================
  Files         245      245              
  Lines       18542    18566      +24     
  Branches      613      602      -11     
==========================================
- Hits        12337    12335       -2     
- Misses       6205     6231      +26     

Impacted Files	Coverage Δ
...state/interpreter/PrecompiledScriptProcessor.scala	98.11% <ø> (ø)
core/src/main/scala/scalan/Base.scala	71.26% <29.62%> (-5.16%)	⬇️
...ate/src/main/scala/sigmastate/eval/IRContext.scala	89.47% <100.00%> (-0.27%)	⬇️
...tate/src/main/scala/sigmastate/utils/Helpers.scala	54.41% <100.00%> (+2.10%)	⬆️
.../src/main/scala/special/collection/ViewColls.scala	87.91% <0.00%> (-1.10%)	⬇️
...ain/scala/special/collection/CollsOverArrays.scala	86.48% <0.00%> (-0.61%)	⬇️
...c/main/scala/special/sigma/impl/SigmaDslImpl.scala	46.38% <0.00%> (-0.23%)	⬇️
...cala/sigmastate/serialization/TypeSerializer.scala	96.55% <0.00%> (ø)

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