Implement Tasty Pickling Independent of the Compiler

[bishabosha]

based on #7616

Draft because currently this uses dotty syntax - which would need to be fixed if tasty-core must cross-compile with scala 2, however hopefully we can benchmark the changes.

I also intend that more things can be abstracted behind the Tasty interface to make its footprint smaller, but for now as few changes as possible were made to the original code

[bishabosha]

test performance please

[dottybot]

performance test scheduled: 3 job(s) in queue, 1 running.

[dottybot]

Performance test finished successfully:

Visit http://dotty-bench.epfl.ch/7659/ to see the changes.

Benchmarks is based on merging with master (3cc436a)

[bishabosha]

test performance please

[dottybot]

performance test scheduled: 2 job(s) in queue, 1 running.

[dottybot]

Performance test finished successfully:

Visit http://dotty-bench.epfl.ch/7659/ to see the changes.

Benchmarks is based on merging with master (3cc436a)

[bishabosha]

so the benchmark graph suggests that this technique had no impact on performance. However the interface is too rigid

[bishabosha]

I am closing this due to all the changes to TASTy that would need updating.

Takeaways:

• pickling is not super important to make generic: consumers of tasty are more widespread
• There should be a low level api for byte manipulation, which specific implementations can tailor to their needs.
• Instead of making a very rigid API to enforce a structure of trees, there should be a standard implementation that other implementations verify themselves against