Cosmetic compiler cleanup (#12718)

* Cleanup compiler code base

* Unify add calls

* Unify len invocations

* Unify range operators

* Fix oversight

* Remove {.procvar.} pragma

* initCandidate -> newCandidate where reasonable

* Unify safeLen calls

compiler/plugins/itersgen.nim

@@ -42,5 +42,5 @@ proc iterToProcImpl*(c: PContext, n: PNode): PNode =
               pattern = c.graph.emptyNode, genericParams = c.graph.emptyNode,
               pragmas = orig[pragmasPos], exceptions = c.graph.emptyNode)
 
-  prc.ast.add iter.sym.ast.sons[resultPos]
+  prc.ast.add iter.sym.ast[resultPos]
   addInterfaceDecl(c, prc)

compiler/plugins/locals.nim

@@ -32,7 +32,7 @@ proc semLocals*(c: PContext, n: PNode): PNode =
           field.position = counter
           inc(counter)
 
-          addSon(tupleType.n, newSymNode(field))
+          tupleType.n.add newSymNode(field)
           addSonSkipIntLit(tupleType, field.typ)
 
           var a = newSymNode(it, result.info)