Semantics of bulk memory operations

[syg]

The semantics of bulk memory operations on shared linear memory seems to be currently underspecified. If the memory model is mostly the same as JS's, I assume these should be unordered atomic.

AFAICT V8's current implementation is also unsafe in that it just calls out to C memmove and memcpy under the hood, which are non-atomic.

@conrad-watt should know best.