VM: support importc var, ptr/pointer types, cast int <=> ptr/pointer (#12877)

* VM: allow certain hardcoded special var variables at CT
* VM: allow all importc var, cast[int](ptr)
* fix tests tests/vm/tstaticprintseq.nim, tests/cpp/t8241.nim
* VM: == works for ptr/pointer nodes
* bugfix: ==, cast now also works for pointer, not just ptr
* VM supports cast PtrLikeKinds <=> PtrLikeKinds / int
* improve cname handling
* fixup + bug fix
* VM: support cast from ref to int
* address comment: opcLdGlobalDeref => opcLdGlobalDerefFFI
* defensive check against typ == nil

compiler/cgen.nim

@@ -700,7 +700,7 @@ proc loadDynamicLib(m: BModule, lib: PLib) =
 
 proc mangleDynLibProc(sym: PSym): Rope =
   # we have to build this as a single rope in order not to trip the
-  # optimization in genInfixCall
+  # optimization in genInfixCall, see test tests/cpp/t8241.nim
   if sfCompilerProc in sym.flags:
     # NOTE: sym.loc.r is the external name!
     result = rope(sym.name.s)