Reland "[vm] MemoryCopy instruction for copying between typed data and strings."

This is a reland of 6ecd8a10ea9fee5a43f8f6a453277d174c4d268d

Original change's description:
> [vm] MemoryCopy instruction for copying between typed data and strings.
> 
> Used for copying the bytes from the Uint8List to the _OneByteString in
> String.fromCharCodes and the pure-ASCII case of UTF-8 decoding.
> 
> Issue https://github.com/dart-lang/sdk/issues/42072
> Closes https://github.com/dart-lang/sdk/issues/41703
> 
> Change-Id: I1ae300222877d1c6e64e32c2f40b8fb187a584c0
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/149500
> Commit-Queue: Aske Simon Christensen <askesc@google.com>
> Reviewed-by: Martin Kustermann <kustermann@google.com>

Change-Id: Ia231c521e5f2db168cfc6094dfc7322327dedc6d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/150925
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Aske Simon Christensen <askesc@google.com>