make Nim take roughly 100MB less RAM for bootstrapping via a new compiler switch -d:leanCompiler; useful for the Raberry PI

compiler/sempass2.nim

@@ -9,9 +9,12 @@
 
 import
   intsets, ast, astalgo, msgs, renderer, magicsys, types, idents, trees,
-  wordrecg, strutils, options, guards, writetracking, lineinfos, semfold,
+  wordrecg, strutils, options, guards, lineinfos, semfold,
   modulegraphs
 
+when not defined(leanCompiler):
+  import writetracking
+
 when defined(useDfa):
   import dfa
 
@@ -713,7 +716,7 @@ proc track(tracked: PEffects, n: PNode) =
       track(tracked, n.sons[i])
   of nkCallKinds:
     if getConstExpr(tracked.owner_module, n, tracked.graph) != nil:
-      return 
+      return
     # p's effects are ours too:
     var a = n.sons[0]
     let op = a.typ