core: add private::exchange_alloc::{malloc_raw,free_raw}

thin wrappers around libc::{malloc,free}, respectively. If/when box headers
go away (and the requirement in exchange_alloc::malloc for a *TypeDesc),
malloc/free and malloc_raw/free_raw can be merged.
core: add libc::wcslen()
core: more work on win32 libc:: bindings for os::list_dir
rt: a few helper functions to facilitate os::list_dir on win32
core: complete porting os::list_dir to rust on win32
rt: dead code cleanup.. no more make_str/make_str_vec

.. that many less calls to kernel->malloc! hooray!