Make dotc faster

[odersky]

This issue collects items that should be looked at in the work to improve performance of dotc.

The following items should definitely be worked on:

• [x] Port the improvements to classpath handling from scalac 2.13
• [x] Port the improvements to classfile parsing from scalac 2.13
• [x] Write class files in parallel, like scalac 2.13
• [x] Finish the implementation of function specialization. Profiles show that we get quite a bit of unexpected boxings in the compiler due to the fact that function types are not yet specialized.
• [x] Go through the previous benchmark results and identify the (top 20-30) commits that caused a performance loss.

The following items are more for exploration:

• [ ] See whether we can replace some usages of lists with arrays, with the aim of getting better locaility
• [ ] See whether we can parallelize more code