Optimization: Introduce CompactAnnotation

[odersky]

This one takes a type argument instead of as a tree argument. This makes mapping such annotations a lot faster and also safer. In fact, in retrospect, most annotations could be represented as CompactAnnotations.

CompactAnnotation is extended by RetainingAnnotation, which is reserved for @retains, @retainsByName and @retainsCap. We make sure that all annotations with these classes are represented as RetainingAnnotations.

For now there are no CompactAnnotations other than @RetainingAnnotations but this could be changed in the future.

[odersky]

I did some benchmarking: I compiled repeatedly the contents of typer/*.scala. Results are here:

Benchmark setup: 2 compiler instances with 20 runs each. Classpath is kept in memory between runs. Tests were run on an M4 Macbook Pro.

lines: Total number of lines
best: average of 13 best runs (out of 40 total)
avg: average of 13 last runs

The gist is:

• Raw compile speed is between 20K and 24K lines / second.
• We lose about 5% between 3.3.6 and 3.6.2.
• We lose a further 10% between 3.6.2 and 3.7.3/3.8.0
• We regain about 1.5% with this PR.

We should drill deeper and find out where the other losses came from.

[odersky]

I noticed that some of the versions in the benchmarks produced warnings and other did not, so I reran them with warnings off. Here are the new numbers.

So, we lose about 4% from 3.3 to 3.6, another 2.5% from 3.6 to 3.7, and another 4% between 3.7 and 3.8. And this PR makes no difference to the 3.8 baseline.