use GC.malloc_atomic to allocate objects without pointers

[kostya]

bdwgc recommend to use malloc_atomic (https://github.com/ivmai/bdwgc#the-c-interface-to-the-allocator) to allocate binary data. I apply this to some places, but there is actually much more places, that can be optimized. Bytes.new for example, or better Pointer(T).malloc choose to use atomic if T is not reference.

[asterite]

I've thought about making Pointer(T).malloc use GC.malloc_atomic if T doesn't have references (the check is trivial for primitive types but a bit more tricky for other types, for example structs). The only gotcha would be to do something like Pointer(UInt8).malloc(size).as(SomeReferenceType) but for those cases we can probably use Pointer(Void) and document that Void should be used in those cases (and we consider Void as potentially having inner pointers).