Change builtin functions mangling and their return type flattening

• the new mangling is more consistent with the language itself
• this allows more user-friendly error messages

added 3 commits

• 612e60d8...776199d4 - 2 commits from branch develop
• 2efad91d - Merge branch 'develop' into feature/builtin-function-polymorphism

Compare with previous version

enabled an automatic merge when the pipeline for 2efad91d succeeds

merged

mentioned in commit 134e340c