[vm] Use specialized int set in heap snapshot tool

Currently the heap snapshot tool uses a regular Set<int> for storing
ints between 0 and the number of objects (n).

This CL instead introduces a specialized int set backed by a Uint8List
where each bit represents a number between 0 and n.

This specialized set is then created to fit the number of objects, so
even an empty set takes the same amount of space as a full set: about
125 kb per million possible values (or ~6 mb per 50 million).

For small sets thats a lot worse than before, but for big sets it's a
lot better. Runtime is also a lot better.

"Benchmarks" on a snapshot that's ~1.5GB:

Runtimes:

Before:
closure roots (or all after the first one): ~30s, ~19s, ~20s
eval and: ~5.5s, ~6.3, ~5.6
eval or: ~7.3s, ~6.5s, ~6.4s
eval minus: ~4.5s, ~6.4s, ~6.5s

With CL:
closure roots (or all after the first one): ~7.7s, ~4.3s, ~4.1s
eval and: ~0.2s, ~0.3s, ~0.2s
eval or: ~0.6s, ~0.6s, ~0.6s
eval minus: ~0.3s, ~0.2s, ~0.3s


Memory usage:

Before:

after loaded:
$ grep -E "VmPeak|VmSize|VmRSS" /proc/588700/status
VmPeak: 10293036 kB
VmSize:  9896880 kB
VmRSS:   9260604 kB

after all = closure roots
VmPeak: 12354840 kB
VmSize:  9529612 kB
VmRSS:   8898088 kB

after 2 x closure all
VmPeak: 13068580 kB
VmSize: 12478236 kB
VmRSS:  11891620 kB

after 3 x eval and
VmPeak: 18377540 kB
VmSize: 18376520 kB
VmRSS:  17803032 kB

after 3 x eval or
VmPeak: 19621712 kB
VmSize: 18114376 kB
VmRSS:  17491808 kB

after 3 x eval minus
VmPeak: 21522780 kB
VmSize: 21522272 kB
VmRSS:  20907724 kB


With CL:

after loaded:
$ grep -E "VmPeak|VmSize|VmRSS" /proc/594718/status
VmPeak: 10293560 kB
VmSize:  9967036 kB
VmRSS:   9323404 kB

after all = closure roots
VmPeak: 10293560 kB
VmSize:  9805272 kB
VmRSS:   9137364 kB

after 2 x closure all
VmPeak: 10868740 kB
VmSize: 10867208 kB
VmRSS:  10199784 kB

after 3 x eval and
VmPeak: 10906236 kB
VmSize: 10905728 kB
VmRSS:  10238424 kB

after 3 x eval or
VmPeak: 10925496 kB
VmSize: 10924988 kB
VmRSS:  10257388 kB

after 3 x eval minus

VmPeak: 10944756 kB
VmSize: 10944248 kB
VmRSS:  10276672 kB
Change-Id: I8d0b65fa51ac2bd4696c4d2782c4423966a7fbae
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/324682
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>