Compare values in TreeMap's 'lt' function

Closes #5194
Small documentation changes

I'm leaving the Sized kind undocumented since it isn't fully implemented
yet.
GC static_size_of_enum, which was unused
Fixed documentation for finalize->drop change
Removing the rename warning

Now that the finalize->drop rename has happened, this hopefully
shouldn't end up changing again.
auto merge of #7549 : sfackler/rust/docs, r=msullivan

I'm leaving the Sized kind undocumented since it isn't fully implemented
yet.
auto merge of #7530 : alexcrichton/rust/issue-5194, r=thestinger

Closes #5194
auto merge of #7554 : jld/rust/rm-ssoe, r=catamorphism
vec: make vec_reserve_shared_actual private
add a temporary vector_exchange_malloc lang item
remove headers from exchange allocations