Add debug_assert and debug_assert_eq macros

I also switched some `assert!` calls over to `debug_assert!`.

Closes #12049.

RFC: 0015-assert
Remove useless assert! case
auto merge of #13789 : sfackler/rust/debug-assert, r=pcwalton

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.

RFC: 0015-assert