symmetric-coroutine: placement new + StackAllocator

- for symmetric_coroutine<> StackAllocator is type-erased
- symmetric_coroutine_impl<> is allocated via placement-new on
  coroutine's stack -> no extra jump to coroutine-context
small changes related to symmetric-coroutine
asyymmetric-coroutine with placement new
dd missing keyword typename
update performance.qbk
add missing stack_allocator parameter to ctor
fix typo: push_coroutine
disable warnings for MSVC
remove tempalte<> for MSVC
Merge branch 'develop'