heap: rm out-of-date FIXMEs
heap: optimize EMPTY to avoid relocations

Sized deallocation makes it pointless to provide an address that never
overlaps with pointers returned by an allocator. Code can branch on the
capacity of the allocation instead of a comparison with this sentinel.

This improves the situation in #8859, and the remaining issues are only
from the logging API, which should be disabled by default in optimized
release builds anyway along with debug assertions. The remaining issues
are part of #17081.

Closes #8859
remove the closure_exchange_malloc lang item
auto merge of #17280 : thestinger/rust/heap, r=pcwalton