better run [feature] (#11709)

* track the checksums of all involved Nim files for smarter 'nim c -r' recompiles
* don't recompile unless necessary for 'nim c -r' [feature]
* [feature] koch boot uses a two step process in order to free the RAM before the GCC/Clang invocations
* fixes a serious regression

koch.nim

@@ -308,8 +308,14 @@ proc boot(args: string) =
         extraOption.add " -d:nimBoostrapCsources0_19_0"
         # remove this when csources get updated
 
-    exec "$# $# $# $# --nimcache:$# compiler" / "nim.nim" %
+    # in order to use less memory, we split the build into two steps:
+    # --compileOnly produces a $project.json file and does not run GCC/Clang.
+    # jsonbuild then uses the $project.json file to build the Nim binary.
+    exec "$# $# $# $# --nimcache:$# --compileOnly compiler" / "nim.nim" %
       [nimi, bootOptions, extraOption, args, smartNimcache]
+    exec "$# jsonscript --nimcache:$# compiler" / "nim.nim" %
+      [nimi, smartNimcache]
+
     if sameFileContent(output, i.thVersion):
       copyExe(output, finalDest)
       echo "executables are equal: SUCCESS!"