[vm] Compact serialization for canonical sets

This CL changes how canonical sets for some specific types are written
into the root snapshot: instead of writing canonical set as a separate
object we reorder objects within a canonical cluster in such a way that
the order matches order of elements in the backing store of a canonical
set and then we write canonical set layout out using differential
encoding (essentially writing gaps between elements instead of writing
absolute indices).

This significantly reduces the overhead of having canonical sets in the
snapshot while maintaining fast deserialisation: for example on build
microbenchmark this brings regression in the snapshot size from 4% to
.3%.

On sizeopt benchmarks:

  flutter_gallery_app_so_gzip_size   -1.5%
  flutter_gallery_app_so_size        -4.7%
  flutter_gallery_total_heap_size   -16.2%


TEST=ci

Change-Id: I2be7fd073668e9b52098e2acda9f11d128cfda95
Cq-Include-Trybots: luci.dart.try:vm-kernel-precomp-dwarf-linux-product-x64-try,vm-kernel-precomp-linux-release-x64-try,vm-kernel-precomp-linux-debug-x64-try,pkg-linux-release-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/185381
Commit-Queue: Vyacheslav Egorov <[email protected]>
Reviewed-by: Siva Annamalai <[email protected]>

Author: mraleph