[kernel] Change constant references; remove need for map

Previously constants were referenced via there relative binary offset,
i.e. a constant reference saying 42 meant at byte offset 42 relative to
the start of the constant table in the binary. This was done to be able
to (on the VM side) read the needed constants lazily. It meant, though,
that constants had to be stored in a map, mapping from the byte position
to the constant.

This change adds a level of indirection when needing the lazy reading,
but lets the constant references reference the constant number instead
so that a constant reference saying 42 means constant number 42,
i.e. constants can be stored in a list instead of in a map.
This is done on the dart side, but the VM still stores it in a map.

The level of indirection is a tabel next to the constant table where
each entry has constant size (4 bytes) from which one can read the
relative byte offset into the constant table from the constant number,
thus still being able to read needed constants lazily.

This CL also cleans up a leftover where for instance double constants
had their textual representation saved as a string in the string indexer
(and thus the output dill) even though they were never referenced.

File size changes:
* Platform: increses 7,816 bytes.
* Compile of dart2js (including platform): decreases 71,424 bytes.

Speed changes:
* Adding `UserTag`s to the code and looking at observatories cpu profile
  on a `pkg/kernel/test/binary_bench.dart --golem AstFromBinaryLazy`
  run of a compile of dart2js reading the constant table has gone
  from ~10% to ~5%.
* Doing statistics on
  `pkg/kernel/test/binary_bench.dart --raw AstFromBinaryLazy` run of a
  compile of dart2js says -6.28169% +/- 4.97269%.
* Golem runs has nothing above the noise level.
  It does say "AstFromBinaryLazy (Intel Xeon) -4.006% (0.4 noise)" and
  "AstFromBinaryEagerP50 (Intel Core i5) -8.929% (0.6 noise)" though.

TEST=All tests running the VM tests this.

Change-Id: I07ead457527a4477de803ee55ba742f5557413d3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/196925
Commit-Queue: Jens Johansen <jensj@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>