[vm] Serialize strings compactly, instead of as heap images.
[vm] Serialize strings compactly, instead of as heap images.

Avoids the space overhead for the class, hash and alignment padding.

Flutter Gallery ARM64 release
Total(CodeSize): 8537094 -> 8088430 (-5.25%)

Change-Id: I73bc984f157042824c29efc42d45547914bd3c1b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/160183
Commit-Queue: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>
Reviewed-by: Tess Strickland <cce218d042444252b7ac446d60910fca7732559c@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk