Remove rust_exchange_alloc

[brson]

This is the C++ implementation of the exchange allocator. It has no users. The only thing that can't be removed yet is the global exchange_count variable.

[yichoi]

It looks to me that rust_exchange_alloc used by reserve_vec_exact in rust_util.h,
reserve_vec_exact used by vec_reserve_shared_actual in rust_builtin.cpp. and so forth.

Does it really has no users ?

[graydon]

Yeah, the vec paths still use it

[thestinger]

I moved the vector reserve for exchange allocations to libstd, so there's not much left using this. I removed the exchange_count atomic counting because valgrind covers this well and we have valgrind bots on auto. It's a noticeable performance hit since it results in a cache miss.

[thestinger]

This is completed.