Add debug_assert and debug_assert_eq macros

I switched the `assert!` calls in `RefCell` over to `debug_assert!`.
There are probably other instances that should be converted as well, but
I couldn't think of any off the top of my head.

rust-lang/rfcs#50 needs to be updated before this PR can be merged but I believe this is the naming that's been settled on.

RFC: XXXX-assert