optimize _memCopy from dart:ffi

[mraleph]

dart:ffi uses the _memCopy helper when working with structs.

We should make sure to

• Provide fast recognized implementation for this method (probably bottoming in MemoryCopyInstr.
• Small constant size copies and unroll the copying loop in place.

[dcharkes]

Duplicate of:

• [vm/ffi] Optimize memcopy in nested struct assignment #43967