exploit nil seqs for performance (#8688)

* changes $ for seqs to never produce 'nil'
* exploit the fact that empty seqs don't have to allocate in the code generator

tests/parallel/tsendtwice.nim

@@ -1,11 +1,11 @@
 discard """
-  output: '''obj2 nil
-obj nil
-obj3 nil
+  output: '''obj2 @[]
+obj @[]
+obj3 @[]
 3
-obj2 nil
-obj nil
-obj3 nil'''
+obj2 @[]
+obj @[]
+obj3 @[]'''
   cmd: "nim c -r --threads:on $file"
 """
 

tests/system/tostring.nim

@@ -24,7 +24,7 @@ doAssert "nan" == $(0.0/0.0)
 # nil tests
 # maybe a bit inconsistent in types
 var x: seq[string]
-doAssert "nil" == $(x)
+doAssert "@[]" == $(x)
 
 var y: string
 doAssert "" == $(y)