core: Move mkdir_recursive from rustpkg into core::os

mkdir_recursive creates a directory as well as any of its
parent directories that don't exist already. Seems like a useful
thing to have in core.
auto merge of #6082 : catamorphism/rust/mkdir_recursive, r=brson

r? @brson mkdir_recursive creates a directory as well as any of its
parent directories that don't exist already. Seems like a useful
thing to have in core.

(Or r? anyone who gets to it first.)