RefCell causes unnecessary code bloat + branches

[thestinger]

It performs unnecessary assertions at the end of every borrow, and these are never disabled.

[sfackler]

Should be fixable once #12108 lands. Alternatively, the assertions could just be removed.

[nikomatsakis]

I'd prefer to keep the assertions (but probably compile them out in non-debug builds). Unsafe code should have plenty of assertions, imo. Helps the reader. Helps the refactorer. Helps everybody.

[gereeter]

I believe this was fixed by #13789 and should be closed.