[pmc2c] support RETURN(PMC* const ...) and (INTVAL pos>>32)

const is no variable name.
allow full expressions as return value, not just names.

just skip ptr dereferences /^\**/. Note that the * return syntax is flawed,
as we should support both cases, return *ptr; and return ptr;
But we get away with it as we return only return (decl)*ptr and return (decl*)ptr;
[test] move test_gc to the end of t/pmc/fixedstringarray.t

all other tests pass fine, even with wrong write barriers
[tools] add tools/dev/vgp_old using parrot_old

building parrot_old is much faster than parrot, when tracing leaks
[gc] simplify the stack scanning loop

just inc the stack ptr
reverse the hi <=> lo markers to make sense
comment the unsafe ptr dereference here, where asan and valgrind do complain.
Revert "[pmc] task: WB related errors"

This reverts commit ab201b1657795217c95c7f62c8f1bf71418687b0.
[pmc] Fixup most WBs. :no_wb for all non-writers, ... [1/2]

:manual_wb on VTABLE method calls on SELF which to a WB (avoid duplicates).
See new description at https://github.com/parrot/parrot/issues/1069
[pmc] Fixup most WBs [2/2]

:no_wb on non-writers, :manual_wb on VTABLE method calls on SELF which to a WB (avoid duplicates).
See new description at https://github.com/parrot/parrot/issues/1069
Some UNUSED missing.

Still the same 2 regressions:
t/op/gc.t                                 (Wstat: 11 Tests: 19 Failed: 0)
  Non-zero wait status: 11
  Parse errors: No plan found in TAP output
t/pmc/namespace-old.t                     (Wstat: 1024 Tests: 38 Failed: 4)
  Failed tests:  27-30
  Non-zero exit status: 4
[pmc] add more UNUSED for :no_wb methods

also remove a wrong task WB