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.

src/rt/rust_kernel.cpp

@@ -79,11 +79,6 @@ rust_kernel::malloc(size_t size, const char *tag) {
     return exchange_alloc.malloc(size);
 }
 
-void *
-rust_kernel::calloc(size_t size, const char *tag) {
-    return exchange_alloc.calloc(size);
-}
-
 void *
 rust_kernel::realloc(void *mem, size_t size) {
     return exchange_alloc.realloc(mem, size);