feat(init functions): struct support

[mhasel]

blocked by #1259

This PR adds generated init functions for each stateful POU and struct type regardless of whether or not the const_evaluator flagged it, resulting in changes to around 400 snapshots. Unfortunately this is unavoidable. I have committed all updated snapshots in a single commit for filtering convenience. Additionally, since each compilation now results in at least 3 compilation units, the test runners had to be updated to allow for "multi-unit" unit-testing. Compiling with -c now compiles to a single module before persisting to object and no longer defaults to linking with the default linker when dealing with more than a single .o-file (caused problems on windows targets).