big steps torwards an efficient, simple IC implementation (#16543)

* reworked ID handling
* the packed AST now has its own ID mechanism
* basic serialization code works
* extract rodfiles to its own module
* rodfiles: store and compare configs
* rodfiles: store dependencies
* store config at the end
* precise dependency tracking
* dependency tracking for rodfiles
* completed loading of PSym, PType, etc
* removed dead code
* bugfix: do not realloc seqs when taking addr into an element
* make IC opt-in for now
* makes tcompilerapi green again
* final cleanups

Co-authored-by: Andy Davidoff <github@andy.disruptek.com>

compiler/commands.nim

@@ -31,7 +31,6 @@ import
   wordrecg, parseutils, nimblecmd, parseopt, sequtils, lineinfos,
   pathutils, strtabs
 
-from incremental import nimIncremental
 from ast import eqTypeFlags, tfGcSafe, tfNoSideEffect
 
 # but some have deps to imported modules. Yay.
@@ -799,10 +798,6 @@ proc processSwitch*(switch, arg: string, pass: TCmdLinePass, info: TLineInfo;
     helpOnError(conf, pass)
   of "symbolfiles": discard "ignore for backwards compat"
   of "incremental":
-    when not nimIncremental:
-      localError(conf, info, "the compiler was not built with " &
-        "incremental compilation features; bootstrap with " &
-        "-d:nimIncremental to enable")
     case arg.normalize
     of "on": conf.symbolFiles = v2Sf
     of "off": conf.symbolFiles = disabledSf