stop initializing ref_count for stack closures

the only part of the header ~fn and &fn should be touching right now is
the tydesc
rm unused visit_str method from TyVisitor
rm unnecessary stage0 `zero_memory` fn
fix repr of unique vectors with stage0 libstd

Closes #7860
fix repr of @mut vectors
repr: add a test case for @mut inside another type
auto merge of #7842 : thestinger/rust/closure, r=huonw