Add Memset+Memcpy to FutureFeatures.md (#1057)

Author: binji

FutureFeatures.md

@@ -561,3 +561,22 @@ static signature validation check. This could be improved by allowing:
   check of a call to a heterogeneous table;
 * any other specific GC reference type, effectively allowing WebAssembly code
   to implement a variety of rooting API schemes.
+
+### Memset and Memcpy Operators
+
+Copying and clearing large memory regions is very common, and making these
+operations fast is architecture dependent. Although this can be done in the MVP
+via `i32.load` and `i32.store`, this requires more bytes of code and forces VMs
+to recognize the loops as well. The following operators can be added to improve
+performance:
+
+* `move_memory`: Copy data from a source memory region to destination region;
+   these regions may overlap: the copy is performed as if the source region was 
+   first copied to a temporary buffer, then the temporary buffer is copied to
+   the destination region
+* `set_memory`: Set all bytes in a memory region to a given byte
+
+We expect that WebAssembly producers will use these operations when the region
+size is known to be large, and will use loads/stores otherwise.
+
+TODO: determine how these operations interact w/ shared memory.