IC: next steps (#16729)

* IC: dead code elimination pass
* preparations for a different codegen strategy
* added documentation to the newly written code
* IC: backend code
* IC: backend adjustments
* optimized the compiler a bit
* IC: yet another massive refactoring
* fixes regressions
* cleanups

compiler/semdata.nim

@@ -558,6 +558,7 @@ proc saveRodFile*(c: PContext) =
         addPragmaComputation(c, n)
     if sfSystemModule in c.module.flags:
       c.graph.systemModuleComplete = true
+    c.idgen.sealed = true # no further additions are allowed
     if c.config.symbolFiles != stressTest:
       # For stress testing we seek to reload the symbols from memory. This
       # way much of the logic is tested but the test is reproducible as it does