Implement SIP-61 `@unroll` annotation

[bishabosha]

~Still need to write documentation,~ Documentation is written also

I am doing this work on behalf of @lihaoyi

The main implementation follows com-lihaoyi/unroll but with some changes:

• @unroll annotation is @experimental
• run before pickling
• increase validation checks for incorrect usage of @unroll
• Underscore not EmptyTree in pattern match default case
• ensure correct spans in TASTy
• ensure symbols are correctly substituted in types

There is one main library addition: scala.annotation.unroll, i.e. the @unroll annotation that appears on parameters,

commits are a bit of a mess - a lot of churn

Edit: removed changes:

• ~reuse the symbol when generating the "forwarder" of an abstract method.~
• ~infer override when overriding trait methods that "appear" abstract in source code, but were actually implemented by the unroll annotation~
• ~do not add the Invisible flag to abstract unrolled methods - this means they can actually still be visible in separately compiled compilation units (because unrolling runs before pickler now)~
• ~Internal annotation scala.annotation.internal.AbstractUnroll - marker used to indicate that an unrolled abstract method was abstract in source code (unrolling phase actually provides a default implementation) - this enables downstream implementers to avoid the override flag. (Maybe we shouldn't allow this convenience?)~
• ~Internal annotation scala.annotation.internal.UnrollForwarder - marker on any generated forwarder - which is used for special casing TASTy pickling of method calls to the forwarders~
• ~because forwarders get the Invisible flag, this makes them impossible to select from TASTy normally (via SELECTin), (invisible def are filtered out "before typer") so I intercept the Select trees to be TERMREFdirect, and then restore them to Select after TASTY. perhaps someone has a better idea, or we could change the resolution rules for Invisible? or invent a new TASTy node? (I also tried generating a Ident tree rather than a Select, but this had a type error)~

fixes #21728