stop zeroing all exchange allocations
rm unused zero param in C++ exchange allocator
fix bug in uv_ll tests uncovered by not zeroing
get rid of unused exchange_alloc calloc method

this isn't actually calloc - it calls the malloc wrapper which no
longer zeroes
rm the unused calloc wrapper from memory_region

it doesn't actually call calloc, so it's fairly pointless
auto merge of #4938 : thestinger/rust/no_zero, r=brson

I removed the unused wrappers methods named `calloc` because they relied on the malloc wrapper having a `bool zero = true` default parameter (which resulted in some accidental zeroing). Perhaps wrapping the actual calloc function would be useful, but I don't know of an existing use case that could use it so I just removed these.

This gives an ~1% performance improvement for TreeMap, which does a lot of small allocations. Vectors use `realloc` which didn't zero before these changes so there's no measurable change in performance.