Unify Scala hashing algorithms

[scabug]

The hashing algorithms in
FlatHashTable and HashTable differ from HashMap and HashSet - and we probably have other variants also.

They should be unified, since HashTable explicitly deprecated the methods used in other places.

See https://issues.scala-lang.org/browse/SI-10049?focusedCommentId=75931 for further details.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-10084?orig=1
Reporter: @paplorinc

[magnolia-k]

Since the FlatHashTable is deleted by the following commit, can this issue be closed?

scala/scala@3a6f637

[SethTisue]

the “we probably have other variants also” part gives me pause — @szeiger @Ichoran @julienrf wdyt?

[eed3si9n]

• scala.util.hashing.ByteswapHashing / scala.util.hashing.byteswap32 / scala.util.hashing.byteswap64
• scala.util.hashing.MurmurHash3
• scala.collection.Hashing.computeHash

[szeiger]

The title is a bit misleading. It's not really about different hashing algorithms (we use .## or .hashCode everywhere) but about improving hashes and mapping them to hash buckets. We still have different algorithms in mutable and immutable hash-based collections. See #11326 for a related discussion.

[magnolia-k]

If the issue is that the right hash algorithm has been chosen, is it better to close the issue itself?
(Or I think that rewriting the title ... unifying is not the point of discussion)