Currently all core and std macros are automatically added to the prelude
via #[macro_use]. However a situation arose where we want to add a new macro
`assert_matches` but don't want to pull it into the standard prelude for
compatibility reasons. By explicitly exporting the macros found in the core and
std crates we get to decide on a per macro basis and can later add them via
the rust_20xx preludes.
Use `splice` to avoid shifting the other items twice.
Put `extern crate std;` first so it's already resolved when we resolve `::std::prelude::rust_20XX`.
