gc: destructors is beginning to work (#10483)

* kochdocs.nim: code cleanup
* docgen: nicer indentation
* parser.nim: code cleanup
* fixes #10458
* make tests green again
* make =destroy mixins
* gc:destructors: produced C code is almost working
* --gc:destructors simple program compiles (but leaks memory)
* gc:destructors make examples compile in C++ mode
* destructors: string implementation bugfixes
* strs.nim: minor code cleanup
* destructors: builtin seqs are beginning to work
* remove debugging helpers

tests/parser/tprecedence.nim

@@ -40,3 +40,12 @@ proc getX(x: MyObject): lent MyField {.inline.} =
 
 let a = MyObject()
 echo a.getX.b.len
+
+
+# bug  #10458
+template t(x: untyped): untyped = "x"
+
+let
+  aaa = t 2 + 4
+  ccc = t (1, 1) + 6
+  ddd = t [0, 1, 2] + 5