[kernel] Shift up specialized kernel tags

Before this CL kernel tags had specialized tags that was marked by the
single high bit, then using the three lowest bits for a value:

128 + value = 0b10000xxx
136 + value = 0b10001xxx
144 + value = 0b10010xxx

So the numbers from 128 to 151 is taken by this scheme, but because of
the high bit marking stuff being special we can't really use 152-256.

This CL shifts the specialized tags up so it instead uses the 3 highest
bits as a marker while still using the lower 3 bits for the value:

224 + value = 0b11100xxx
232 + value = 0b11101xxx
240 + value = 0b11110xxx

This takes up 224-247 and leave 248-255 unused. It would let us use
128-223 though.
(If we eventually need more we can probably remove one of the
specialized ranges (SpecializedVariableSet isn't used very much in
previously sampled dill files) and use 4 bits for tagging).

Additionally, a tool to print free tags has been added (via binary.md),
and the "binary version is in sync with VM" test has been prepared
for version > 99.

TEST=Existing tests.

Change-Id: If77b12cee6fc3801628dd67dc40afbb018ec8a61
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/284302
Reviewed-by: Martin Kustermann <kustermann@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>